MediaGoblin should show an appropriate, helpful error message when uploading unsupported media type

[Christopher Allan Webber]

Currently if you try to upload a file for a media type that isn't handled / isn't currently registered, you get the 500 error page and in the traceback get:

File '/home/cwebber/devel/mediagoblin/mediagoblin/app.py', line 171 in __call__
  response = controller(request)
File '/home/cwebber/devel/mediagoblin/mediagoblin/decorators.py', line 50 in new_controller_func
  return controller(request, *args, **kwargs)
File '/home/cwebber/devel/mediagoblin/mediagoblin/submit/views.py', line 53 in submit_start
  media_type, media_manager = get_media_type_and_manager(filename)
TypeError: 'NoneType' object is not iterable

This isn't helpful; we should give an error page yes, but one with useful information that says "sorry, we couldn't recognize that media type, it might not be supported or not enabled for this site", or something.

This is a semi-bitesized task.