This takes from the draft spec at https://github.com/lightningnetwork/lightning-rfc/pull/834
Note that if this draft does not get included, the peer will simply
ignore the warning message (we always close the connection afterwards
anyway).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: Protocol: we now report the new (draft) warning message.
