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.
How useful is it for us to define `HandleChannelClose` and `HandleFatalError` as functions attached to the `QueueConnection` struct? I ask because we define them just above and I think it makes the code harder to understand without providing any real benefit as far as I can tell.