If a plugin install fails an error in the exception handling crashes Picard.

Example:

E: 11:31:13,404 /app/lib/python3.5/site-packages/picard/plugin.load_plugin:349: Plugin 'fanarttv' : Traceback (most recent call last):
  File "/app/lib/python3.5/site-packages/picard/plugin.py", line 324, in load_plugin
    plugin_module = importer.load_module(_PLUGIN_MODULE_PREFIX + name)
  File "/home/phw/.var/app/org.musicbrainz.Picard/config/MusicBrainz/Picard/plugins/fanarttv.zip/fanarttv/_init_.py", line 30, in <module>
    from PyQt4.QtCore import QUrl
ImportError: No module named 'PyQt4'

Traceback (most recent call last):
  File "/app/lib/python3.5/site-packages/picard/plugin.py", line 439, in install_plugin
    installed_plugin = self.load_plugin(zip_plugin or plugin_name, USER_PLUGIN_DIR)
  File "/app/lib/python3.5/site-packages/picard/plugin.py", line 297, in load_plugin
    module_pathname = importer.get_filename(name)
zipimport.ZipImportError: bad local file header: '/home/phw/.var/app/org.musicbrainz.Picard/config/MusicBrainz/Picard/plugins/fanarttv.zip'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/app/lib/python3.5/site-packages/picard/webservice/_init_.py", line 426, in _process_reply
    self._handle_reply(reply, request)
  File "/app/lib/python3.5/site-packages/picard/webservice/_init_.py", line 415, in _handle_reply
    handler(reply.readAll(), reply, error)
  File "/app/lib/python3.5/site-packages/picard/ui/options/plugins.py", line 401, in download_handler
    plugin_data=response,
  File "/app/lib/python3.5/site-packages/picard/plugin.py", line 441, in install_plugin
    log.error('Unable to load plugin: %s.\nError occured: %s', name, e)
NameError: name 'name' is not defined