ValueError when opening a read-only hdf twice 
```
In [3]: with pd.HDFStore('/tmp/file_I_can_only_read.hdf') as store:
   ...:     store['test'] = pd.Series(range(3))
   ...:     

In [4]: os.chmod('/tmp/file_I_can_only_read.hdf', 0o400)

In [5]: store1 = pd.HDFStore('/tmp/file_I_can_only_read.hdf')
Opening /tmp/file_I_can_only_read.hdf in read-only mode

In [6]: store2 = pd.HDFStore('/tmp/file_I_can_only_read.hdf')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-5b173bb4a3e8> in <module>()
----> 1 store2 = pd.HDFStore('/tmp/file_I_can_only_read.hdf')

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in __init__(self, path, mode, complevel, complib, fletcher32, **kwargs)
    402         self._fletcher32 = fletcher32
    403         self._filters = None
--> 404         self.open(mode=mode, **kwargs)
    405 
    406     @property

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in open(self, mode, **kwargs)
    563                             hdf_version=tables.get_hdf5_version()))
    564 
--> 565             raise e
    566 
    567         except (Exception) as e:

/home/pietro/nobackup/repo/pandas/pandas/io/pytables.py in open(self, mode, **kwargs)
    538 
    539         try:
--> 540             self._handle = tables.open_file(self._path, self._mode, **kwargs)
    541         except (IOError) as e:  # pragma: no cover
    542             if 'can not be written' in str(e):

/usr/lib/python3/dist-packages/tables/file.py in open_file(filename, mode, title, root_uep, filters, **kwargs)
    308                     "The file '%s' is already opened, but "
    309                     "in read-only mode.  Please close it before "
--> 310                     "reopening in append mode." % filename)
    311             # 'w' means that we want to destroy existing contents
    312             elif mode == 'w':

ValueError: The file '/tmp/file_I_can_only_read.hdf' is already opened, but in read-only mode.  Please close it before reopening in append mode.
```
