Fix log.exception

log.exception() without a message will fail and module build cannot move
to failed state.

Signed-off-by: Chenxiong Qi <cqi@redhat.com>

module_build_service/scheduler/consumer.py

@@ -255,7 +255,7 @@ class MBSConsumer(fedmsg.consumers.FedmsgConsumer):
         try:
             further_work = handler(conf, db_session, msg) or []
         except Exception as e:
-            log.exception()
+            log.exception("Could not process message handler.")
             db_session.rollback()
             db_session.refresh(build)
             build.transition(