Command messages raise IgnoreMessage but only real connectors handle this.

Our command message processing appears to raise ``vumi.connectors.IgnoreMessage`` via ``get_config_for_conversation`` and ``get_config_for_router`` but only real connectors handle this and the custom connectors for command messages don't (so the errors escape unhandled and the consumer stops).
What is this change for? Was the helper doing the wrong thing?