Better handle pika connection errors

It seems like pika may raise AttributeError when the connection is closed,
as describe here: https://github.com/pika/pika/issues/638, resulting
in this error displayed on the webclient:
500: POST /storyboard_api/tasks: 'NoneType' object has no attribute 'send'

This change adds AttributeError to the ConnectionClosed except.

Change-Id: Iffa1ef1998379097b32d699a360b5de9277c01ed

storyboard/notifications/publisher.py

@@ -103,7 +103,7 @@ class Publisher(ConnectionService):
             if payload in self._pending:
                 self._pending.remove(payload)
             return True
-        except ConnectionClosed as cc:
+        except (ConnectionClosed, AttributeError) as cc:
             LOG.warning(_LW("Attempted to send message on closed connection."))
             LOG.debug(cc)
             self._open = False