Raise a special exception for special messages.

Today, we ignore koji messages with a None `task_id` in two ways.  First
with a conditional, and then with a second check in the message
`__init__` method.  This is a belt-and-suspenders approach.

For other reasons in the UMB messaging plugin, we'd like to put this
check in just one place and use a special exception in the initializer
instead of in a conditional beforehand.

module_build_service/messaging.py

@@ -35,6 +35,10 @@ except ImportError:
 from module_build_service import log, conf
 
 
+class IgnoreMessage(Exception):
+    pass
+
+
 class BaseMessage(object):
     def __init__(self, msg_id):
         """
@@ -141,10 +145,6 @@ class FedmsgMessageParser(MessageParser):
                 build_version = msg_inner_msg.get('version')
                 build_release = msg_inner_msg.get('release')
 
-                if task_id is None:
-                    log.debug("Saw a koji build change with no task_id.")
-                    return
-
                 msg_obj = KojiBuildChange(
                     msg_id, build_id, task_id, build_new_state, build_name,
                     build_version, build_release)
@@ -199,7 +199,7 @@ class KojiBuildChange(BaseMessage):
                  build_version, build_release, module_build_id=None,
                  state_reason=None):
         if task_id is None:
-            raise ValueError("KojiBuildChange with a null task_id is invalid.")
+            raise IgnoreMessage("KojiBuildChange with a null task_id is invalid.")
         super(KojiBuildChange, self).__init__(msg_id)
         self.build_id = build_id
         self.task_id = task_id

module_build_service/scheduler/consumer.py

@@ -169,7 +169,10 @@ class MBSConsumer(fedmsg.consumers.FedmsgConsumer):
         parser = module_build_service.messaging.\
                  _messaging_backends[conf.messaging].get('parser')
         if parser:
-            return parser.parse(message)
+            try:
+                return parser.parse(message)
+            except module_build_service.messaging.IgnoreMessage:
+                pass
         else:
             raise ValueError('{0} backend does not define a message parser'
                              .format(conf.messaging))