Issue title: Dropbox: Failed to grab locks
 Issue body: After doing a sync with a large folder (188451 files), I received a few errors (56 files). Out of these few errors most where due to a "Failed to grab locks". I did some research and it looks like this is a generic API error as explained here: https://www.dropboxforum.com/hc/en-us/community/posts/204555365-Failed-to-grab-locks. Their recommended solution is to simply retry the failed files. Does rclone attempt to retry? If not, can it? Here are some samples of the errors I was receiving:

```
2015/08/18 14:17:08 wp-content/uploads/random-photo1.jpg: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 4744768.Please re-issue the request.
2015/08/18 14:19:22 wp-content/uploads/random-pdf.pdf: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 1407014.Please re-issue the request.
2015/08/18 14:19:22 wp-content/uploads/random-photo2.jpg: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 1407014.Please re-issue the request.
2015/08/18 14:37:51 wp-content/plugins/google-analytics-for-wordpress/languages/google-analytics-for-wordpress-pl_PL.mo: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 1406972.Please re-issue the request.
2015/08/18 14:43:39 wp-content/plugins/jetpack/modules/widgets/facebook-likebox/style.css: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 4554587.Please re-issue the request.
2015/08/18 14:57:15 wp-content/plugins/woocommerce/templates/myaccount/form-login.php: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 1407011.Please re-issue the request.
```

 Comments: 
 Comment 0: Thanks for the report.

If you retry the sync it should tidy up the 56 files.

Currently rclone doesn't retry, but I'm considering a --retries flag to automatically retry for every backend.

Also I could make dropbox retry for this specific case - I'll take a look at the API docs.

 Comment 1: After running the same sync multiple times, I was able to slowly lower the error count. After running it 10+ times, I'm still left with ~17 errors. The odd thing is the same files seem to upload even tho the file set is the same. Here is the output of running the same sync 3 times. 

```
Transferred:     10869623 Bytes (  47.46 kByte/s)
Errors:                17
Checks:            188456
Transferred:           50
Elapsed time:  3m43.652556927s

Transferred:      7029029 Bytes (  25.14 kByte/s)
Errors:                17
Checks:            188457
Transferred:           49
Elapsed time:  4m33.065457712s

Transferred:     10862907 Bytes (  47.72 kByte/s)
Errors:                18
Checks:            188457
Transferred:           49
Elapsed time:  3m42.316870934s
```
Here is a full log file. It looks like some persistence file lock errors no matter how many times I run the sync and lots of  .DS_Store is on the ignored file list, so it was not saved. 
```
2015/08/19 04:40:58 Dropbox root 'backup/Sites/anchor.network': Building file list
2015/08/19 04:42:50 Dropbox root 'backup/Sites/anchor.network': Waiting for checks to finish
2015/08/19 04:42:58 Transferring:  wp-content/blogs.dir/103/files/2013/12/STORMWATER-MANAGEMENT-ORDINANCE-501.740.4587.pdf, wp-content/blogs.dir/106/files/2014/01/Cover-to-post-on-blog-150x150.jpg, wp-content/blogs.dir/106/files/2014/01/Cover-to-post-on-blog.jpg, wp-content/blogs.dir/106/files/2014/01/Cover-to-post-on-blog1-150x150.jpg
2015/08/19 04:43:00 wp-content/blogs.dir/106/files/2014/01/Cover-to-post-on-blog.jpg: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 4482258.Please re-issue the request.
2015/08/19 04:43:58 Transferring:  wp-content/blogs.dir/103/files/2013/12/STORMWATER-MANAGEMENT-ORDINANCE-501.740.4587.pdf, wp-content/blogs.dir/127/files/2014/09/DSC_0141.jpg
2015/08/19 04:44:08 wp-content/plugins/advanced-custom-fields-pro/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:10 wp-content/plugins/advanced-custom-fields-pro/admin/updates/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/admin/updates/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:10 wp-content/plugins/advanced-custom-fields-pro/admin/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/admin/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:13 wp-content/plugins/advanced-custom-fields-pro/admin/views/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/admin/views/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:13 wp-content/plugins/advanced-custom-fields-pro/assets/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/assets/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:16 wp-content/plugins/advanced-custom-fields-pro/assets/inc/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/assets/inc/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:16 wp-content/plugins/advanced-custom-fields-pro/assets/css/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/assets/css/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:17 wp-content/plugins/advanced-custom-fields-pro/assets/inc/datepicker/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/assets/inc/datepicker/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:18 wp-content/plugins/advanced-custom-fields-pro/assets/inc/select2/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/assets/inc/select2/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:19 wp-content/plugins/advanced-custom-fields-pro/assets/js/.DS_Store: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 1406973.Please re-issue the request.
2015/08/19 04:44:20 wp-content/plugins/advanced-custom-fields-pro/fields/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/fields/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:20 wp-content/plugins/advanced-custom-fields-pro/forms/.DS_Store: Failed to copy: Upload failed: Failed to grab locks for 586858203: lock held by connection 4483853.Please re-issue the request.
2015/08/19 04:44:21 wp-content/plugins/advanced-custom-fields-pro/lang/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/lang/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:23 wp-content/plugins/advanced-custom-fields-pro/pro/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/pro/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:24 wp-content/plugins/advanced-custom-fields-pro/pro/assets/js/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/pro/assets/js/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:24 wp-content/plugins/advanced-custom-fields-pro/pro/admin/.DS_Store: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/advanced-custom-fields-pro/pro/admin/.DS_Store is on the ignored file list, so it was not saved.
2015/08/19 04:44:25 wp-content/plugins/gravityformspaypalpaymentspro/images/Thumbs.db: Failed to copy: Upload failed: The file /backup/sites/anchor.network/wp-content/plugins/gravityformspaypalpaymentspro/images/Thumbs.db is on the ignored file list, so it was not saved.
2015/08/19 04:44:39 Dropbox root 'backup/Sites/anchor.network': Waiting for transfers to finish
2015/08/19 04:44:39 Dropbox root 'backup/Sites/anchor.network': Not deleting files as there were IO errors
```

 Comment 2: If I try uploading a file called `.DS_Store` through the website I get "Sorry! This file name is not allowed.".  

A bit of searching and this is [the best list I can find of file names which aren't allowed](https://www.dropbox.com/en/help/145) - see Ignored files.  I've also seen people saying to avoid windows tempfile names.

Of 18 errors above, there are 15 to do with the ignored file list.  That leaves 3 lock errors.  However 2 of those are to do with directories called `.DS_Store` so there is actually only 1 file missing from the sync.

Perhaps if rclone ignored the files which dropbox doesn't like that would go through too.

You could always try a more narrow sync to get it to go through, eg

```
rclone -v copy wp-content/blogs.dir/106/files/2014/01/ dropbox:wp-content/blogs.dir/106/files/2014/01/
```

Anyway I think that is two bugs now
- retry the lock fails
- add an exclude list for files dropbox can't upload

 Comment 3: Makes sense. Maybe adding an exclude option like rsync's --exclude? Unless that already exists. 

I synced the individual folder as you recommend. I ran it 4 times with varying outputs. Take a look here: https://cloudup.com/cJ_sUMBbhrH. 
 Comment 4: Ok digging deeper, I think I discovered the root bug. On my file system, it appears I have 2 files with the same name and different file sizes. 
```
-rw-r--r--. 1 root root  59K Jan 25  2015 cover-to-post-on-blog.jpg
-rw-r--r--. 1 root root  32K Jan 25  2015 Cover-to-post-on-blog.jpg
```
Everytime I run the rclone sync to Dropbox it's randomly picking one. Sometimes is deletes the file, sometimes it uploads file 1 and sometimes it upload file 2. Take a look at this file's revisions on Dropbox: https://cloudup.com/ciQJVyK6se7. I'm not sure the best way to handle this considering Dropbox will not store both files. Is it possible to have rclone know if there are multiple files with the same name, different case sensitivity? If so then maybe rclone can attempt the "first" alphabetical order option and ignore the rest.
 Comment 5: I've encountered the initial error when uploading hundreds/thousands of files in one go. Second attempt works OK after that. +1 for adding a "--retries" option though.
 Comment 6: @austinginder well done - good investigation - that sounds spot on.

rclone could give a warning quite easily.  It would be fairly tricky to pick the first alphabetically as the internals of rclone are concurrent and it doesn't actually get the listing then transfer files, it does everything zachary98@example.org.

I'll make a separate ticket for that

 Comment 7: @darren12345 I made a ticket for `--retries` in #109 so I don't forget!

 Comment 8: Super. Although I noticed today you had the forethought to make rclone return a non-zero exit code in the event of any errors so I've been able to wrap it in an while loop.

 

> On 20 Aug 2015, at 16:42, Nick Craig-Wood <zachary98@example.org> wrote:
> 
> @darren12345 I made a ticket for --retries in #109 so I don't forget!
> 
> —
> Reply to this email directly or view it on GitHub.
> 

 Comment 9: Since the lock problem happens very rarely and is difficult to detect, I'm going to implement `--retries` #109 in favour of this ticket.

Since the other issues have been split out into #107 and #108 that leaves nothing left in this issue so I'm going to close it.

Thank you very much for taking the time to report the issues, and do follow the issues above if you are interested.
 Comment 10: Sounds good. Thanks for all your work! It's much appreciated. 
{'source': 'GithubIssues', 'repo_name_with_owner': 'rclone/rclone', 'issue_id': 101730035, 'repo_stars': 35347.0}