bLIP-52 service: Handle more cases of `HTLCHandlingFailed`

[tnull]

Currently, the LSPS2ServiceHandler::htlc_handling_failed only only handles HTLCDestination::NextHopChannel, but it should likely at the very least also handle HTLCDestination::UnknownNextHop and HTLCDestination::InvalidForward variants, i.e., all for which we at least know the intended intercept SCID.