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.
The main reason for exposing them publicly here is so that they can be overriden (and verified) in our tests. If they're not exported, it's a little hard to adjust them to make testing easier. Also, it's a little harder to mute them for our tests.