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.
Merge pull request #48 from alphagov/log-when-the-channel-is-closed

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