HTTP callback fixes

- Added HTTPError catching around the callback request, to not mark the
  entry as failed, just log the exception.
- Fixed bug where I forgot to actually fetch the entry before passing it
  to json_processing_callback.
- Changed __main__ migration #6 to create the ProcessingMetaData table
  as it is currently, to prevent possible breakage if a siteadmin
  is lagging behind with his db migrations and more than one migration
  wants to fix stuff with the ProcessingMetaData table.

mediagoblin/processing/task.py

@@ -96,5 +96,5 @@ class ProcessMedia(Task):
         entry_id = args[0]
         mark_entry_failed(entry_id, exc)
 
-        entry = mgg.database.MediaEntry.query.filter_by(id=entry_id)
+        entry = mgg.database.MediaEntry.query.filter_by(id=entry_id).first()
         json_processing_callback(entry)