Update error handling for outgoing channel not found to catch the case
where an outgoing channel was not found for a failed HTLC. Unlike
incoming HTLCs, where the HTLC arrived on the channel so we know it
exists, we have not yet performed any existence validation on the
outgoing channel (because interception happens before we check that it
exists).
Since we only need the outgoing channel for record keeping, we just
log the case where a HTLC was failed back and we don't know the channel
(since this is just a bogus channel). We don't store this HTLC in the
DB, as it will be instantly failed back.
