If trying to destroy a channel that doesn't exist using socket.destroyChannel, throw clean error or handle gracefully

[jondubois]

Now it throws an error about trying to call unwatch function on undefined.
We need to handle this consistently with the socket.unsubscribe function and others - At the moment they don't throw an error if channel doesn't exist.

[jondubois]

Also see SocketCluster/socketcluster#276

[jondubois]

Done.