fetchart: Handle exceptions in Google backend

[randybias]

With a big library if you force a fetchart there is a good chance that a connection is reset and unfortunately, this case isn't handled with some basic retrying:

fetchart: The Chemical Brothers - The Private Psychedelic Reel: found album art
fetchart: The Chemical Brothers - We Are the Night: found album art
Traceback (most recent call last):
  File "/usr/local/bin/beet", line 9, in <module>
    load_entry_point('beets==1.3.15', 'console_scripts', 'beet')()
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/beets-1.3.15-py2.7.egg/beets/ui/__init__.py", line 1161, in main
    _raw_main(args)
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/beets-1.3.15-py2.7.egg/beets/ui/__init__.py", line 1151, in _raw_main
    subcommand.func(lib, suboptions, subargs)
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/beets-1.3.15-py2.7.egg/beetsplug/fetchart.py", line 412, in func
    self.batch_fetch_art(lib, lib.albums(ui.decargs(args)), opts.force)
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/beets-1.3.15-py2.7.egg/beetsplug/fetchart.py", line 513, in batch_fetch_art
    path = self.art_for_album(album, local_paths)
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/beets-1.3.15-py2.7.egg/beetsplug/fetchart.py", line 487, in art_for_album
    for url in self._source_urls(album):
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/beets-1.3.15-py2.7.egg/beetsplug/fetchart.py", line 532, in _source_urls
    for url in urls:
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/beets-1.3.15-py2.7.egg/beetsplug/fetchart.py", line 125, in get
    'start': '0',
  File "/Library/Python/2.7/site-packages/requests/sessions.py", line 473, in get
    return self.request('GET', url, **kwargs)
  File "/Library/Python/2.7/site-packages/requests/sessions.py", line 461, in request
    resp = self.send(prep, **send_kwargs)
  File "/Library/Python/2.7/site-packages/requests/sessions.py", line 573, in send
    r = adapter.send(request, **kwargs)
  File "/Library/Python/2.7/site-packages/requests/adapters.py", line 415, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', error(54, 'Connection reset by peer'))

[sampsyo]

Thanks. Looks like a bug specifically in the Google backend, which doesn't catch any exceptions for its search request:
https://github.com/sampsyo/beets/blob/master/beetsplug/fetchart.py#L122

We do have error handling for most cases:
https://github.com/sampsyo/beets/blob/master/beetsplug/fetchart.py#L439