Add channel error handler to the agency.

It the agency starts and there are already queues with different parameters then a channel exception is raised. The agency doesn't have an `on_error` handler so the message is lost and it's really, really confusing as to what is going on. The only clue is in the rabbitmq log.

This will be superceded when I rewrite the agency to not use rabbitmq for agency to agent communications.
