Added API traps for RAVEN Superevents.

The server will now return a 400 "bad request" error for RAVEN
createVOEvent calls if:
  1. em_type for the associated superevent is None
  2. em_type for the associated superevent is not a valid graceid
  3. the event associated with em_type cannot be found
  4. the combined_skymap_filename is set, but the file could not
     be found.