Gracefully handle connection/channel termination

[michaelklishin]

One of our (not yet public) test suites uses this plugin to test rabbitmq-plugins. As of rabbitmq/rabbitmq-server#500 and rabbitmq/rabbitmq-server#627, when a vhost is deleted, all connections in it will be force closed. This currently causes this plugin to effectively shut down, messing up certain test expectations because plugin activation never really finishes (!).

[hairyhum]

Direct connections are not being closed during vhost deletion. The plugin shuts down because of a channel error trying to publish to a missing exchange.