print a warning when a message was not sent

this is a partial fix for #48, in that it will print a message if the message will not be sent. this should probably  (partly?) be handled by potr. The source already states "[TODO notify](https://github.com/afflux/pure-python-otr/blob/487f711332dab78f0fbfb28486b70f450fd5ee71/src/potr/context.py#L282)".