HTTP callback fixes

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.