Support reading `channel_update`s in onions with message type bytes

Support reading `channel_update`s in onions with message type bytes

Historically c-lightning and eclair have always sent
`channel_update` messages inside the onion error packets with the
two message type bytes (`0x0102` for `channel_update` messages) but
lnd and us skipped those bytes. We only supported decoding messages
matching our own encoding - decoding a bogus `channel_update` if
the extra bytes were included.

Here we detect the type bytes and, if they're present, start
reading at offset 2.

We also take this opportunity to improve loging and make the
severity more accurate for the previous error.

Fixes #1450.