Correctly set a non-zero exit status when mpirun is terminated by signal

Fixes trac:4537

cmr=v1.8.1:reviewer=jsquyres

This commit was SVN r31423.

The following Trac tickets were found above:
  Ticket 4537 --> https://svn.open-mpi.org/trac/ompi/ticket/4537

orte/mca/ess/hnp/ess_hnp_module.c

@@ -894,6 +894,8 @@ static void clean_abort(int fd, short flags, void *arg)
         opal_event_add(&term_handler, NULL);
         return;
     }
+    /* ensure we exit with a non-zero status */
+    ORTE_UPDATE_EXIT_STATUS(ORTE_ERROR_DEFAULT_EXIT_CODE);
 
     /* ensure that the forwarding of stdin stops */
     orte_job_term_ordered = true;