Revision 36537

  Patch to improve the diagnostic in case of exceptions. Information about the event and file being
  processed at the moment the exception was raised is sent to the client, e.g.

0.5: caught exception triggered by signal '1' while processing dset:'EventTree',
   file:'http://root.cern.ch/files/data/event_3.root', event:1 - check logs for possible stacktrace

  The patch also fixes a problem with submergers observed when a worker was stopped because above
  the memory limits: this worker was established as merger but could not do the work, for obvious
  reasons, freezing the session.