Capture and properly handle errors.

Capture and properly handle errors.

Handled in several places:

 - In the run() of the ProcessMedia itself for
   handled (BaseProcessingFail derived) errors (best to do these not
   in on_failure because the errors are highlighted in celeryd in a
   way that looks inappropriate for when the errors are well handled)
 - In ProcessMedia.on_failure() for all other errors
 - In the submit view where all exceptions are caught, media is marked
   at having failed, then the error is re-raised.  (The reason for
   this is that users running in "lazy" mode will get errors
   propagated by celery and so on_failure won't run for them.)