find_storage_directories: prevent crash with nonexistent directories

[hans]

I ran zotcli configure on OS X using the pip-distributed package and got the following error:

$ zotcli configure
Do you already have an API key for zotero-cli? [y/N]: n
Opening ...
Enter verification code: ...
[0] Local Zotero storage
[1] Use Zotero file cloud
[2] Use WebDAV storage
How do you want to access files for reading? [1]: 0
Traceback (most recent call last):
  File "/Users/jon/anaconda/bin/zotcli", line 11, in <module>
    load_entry_point('zotero-cli==0.3.0', 'console_scripts', 'zotcli')()
  File "/Users/jon/anaconda/lib/python3.5/site-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/Users/jon/anaconda/lib/python3.5/site-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/Users/jon/anaconda/lib/python3.5/site-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/jon/anaconda/lib/python3.5/site-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/jon/anaconda/lib/python3.5/site-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/Users/jon/anaconda/lib/python3.5/site-packages/zotero_cli/cli.py", line 91, in configure
    storage_dirs = tuple(find_storage_directories())
  File "/Users/jon/anaconda/lib/python3.5/site-packages/zotero_cli/cli.py", line 44, in find_storage_directories
    for fpath in candidate_iter:
  File "/Users/jon/anaconda/lib/python3.5/pathlib.py", line 1049, in iterdir
    for name in self._accessor.listdir(self):
  File "/Users/jon/anaconda/lib/python3.5/pathlib.py", line 371, in wrapped
    return strfunc(str(pathobj), *args)
FileNotFoundError: [Errno 2] No such file or directory: '/Users/jon/.mozilla/firefox'

This PR prevents this crash and adds support for the OS X Zotero profile directory.
Thanks for making this package!

[hans]

(I'm not able to test on non-OSX, so let me know if I accidentally broke e.g. the Linux directory setup.)

[jbaiter]

Sorry for ignoring this issue for so long :/
I just merged a fix from @DancingQuanta (#35) that improves discovery of Zotero storage directories, can you try if that works for you?

[BlueDrink9]

Sorry to resurrect, but since the original author never responded, I thought I'd mention that I also have this error, so #35 doesn't seem to have fixed it.