Add Onion failure `update_channel` message type bytes

[TheBlueMatt]

See lightning/bolts#979. Also we currently don't decode such messages, we (really) need to.

[TheBlueMatt]

If you want to work on this, take #1455 and extend it by writing tests and doing the encoding end as well.

[tnull]

As just mentioned in the dev meeting, I'd be up to write some tests on that.