Log when the AMQP channel's `notifyClose` channel is closed

We didn't have anything to check whether the `notifyClose` channel was
open. We were only doing something when an `amqp.Error` message was
added to the channel.

By adding this, we'll be able to track when/why a channel might be
closed.
Makes sense. Does it happen often enough for it to be important to mask them in our tests? It seems a strange to change the code to make testing easier but I could be convinced otherwise :-)