messages provided with a return_msg fail to init (bad keyword): two steps:
1) return_msg is a valid term in the wire protocol, it should be valid to receive it
2) dripline should be forward compatible, by letting messages receive extra keywords and either drop them with a warning, or even just keep them in the message
done, I think that some contents could be even smarter (sender_info should be required in the from_encoded methods, but not allowed (and filled in by local inspection) if creating a new message
messages provided with a return_msg fail to init (bad keyword): two steps: 1) return_msg is a valid term in the wire protocol, it should be valid to receive it 2) dripline should be forward compatible, by letting messages receive extra keywords and either drop them with a warning, or even just keep them in the message