Issue title: mount.s3ql  ERROR: No S3QL file system found at given storage URL.
 Issue body: i use s3ql access openstack swift, authentication is bypass ,but the error occur `ERROR: No S3QL file system found at given storage URL.` it‘s so baffling. 
```shell
mount.s3ql swiftks://116.69.203.115:5000/RegionOne:rayciw /var/srv/node/sl --backend-options no-feature-detection,no-ssl,domain=default,tenant-is-name
{'backend-login': 'admin:admin', 'backend-password': 'admin123'}
Using 10 upload threads.
Autodetected 4034 file descriptors available for cache entries
ERROR: No S3QL file system found at given storage URL.
```

 Comments: 
 Comment 0: im sorry ,i forget exec  mkfs.s3ql before mount.s3ql ,now it works
{'source': 'GithubIssues', 'repo_name_with_owner': 's3ql/s3ql', 'issue_id': 983762308, 'repo_stars': 801.0}