Return 404 (or maybe 204) in /api/series/$id/revisons/$version/mbox if the revision is incomplete

Right now, when the series is imcomplete (because we haven't received all patches for instance), we return a partial mbox.

It'll be nice to not do that so we don't trick the user into believing the series mbox file is valid. Returning 404 or 204 would work. We need to add a per-revision number of patches for that to work though.