[bugfix] don't kill transaction if we don't update last chain height

This changes handler_observed_txin to not error if we are unable to set the last chain height. It is possible that we will be processing transactions out of order of how they were observed on the chain. This change prints the error, but doesn't error the transaction.

Also a little clean up around setting the last block height. We want the set last signed block height to not decrease when being set, as well.