Fail channel if we can't sign a new commitment tx during HTLC claim

Fail channel if we can't sign a new commitment tx during HTLC claim

Previously, we could fail to generate a new commitment transaction
but it simply indicated we had gone to doule-claim an HTLC. Now
that double-claims are returned instead as Ok(None), we should
handle the error case and fail the channel, as the only way to hit
the error case is if key derivation failed or the user refused to
sign the new commitment transaction.

This also resolves an issue where we wouldn't inform our
ChannelMonitor of the new payment preimage in case we failed to
fetch a signature for the new commitment transaction.