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.

[Christopher Allan Webber]

It would be great if that could include an error page, and also if
the function raising the error raised a more appropriate error.

I believe there's some other exception defined in that module meant
to be raised, it should raise that and catch it in the view.

[Elrond]

Can you please give a simple way to reproduce this backtrace?
Like "submit a .XYZ while having media\_types = ... in the config"?
That would help in actually creating a correct error page.

[Will Kahn-Greene]

The original url for this bug was ​http://bugs.foocorp.net/issues/714 .

[Elrond]

We need a proper, failing test case!

[Christopher Allan Webber]

I could add a failing unit test.

I guess it should return the response code "415 Unsupported Media Type"... that's exactly the purpose this http status code was built for :)