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.