deduplicate logging and log transaction ID and amount (#117)

Refactor the logging introduced in !135 (merged) to be a little cleaner. We deduplicate a dual logging.error call, but also include the trxn_id and total_amount metrics in the logs, so that, in normal operation, we can control for abuse without having to retroactively increase the log level (which is impossible).

This matches the data requested by @mattlav in #114 as well, which was "transaction number, amount, status, and timestamp". We don't have the IP address in there, but that's something we can fix later as well.