[FR] Add support for log levels

[yorickdowne]

Feature request: Add log levels, at a minimum info and error. Nice to have: crit (an error that keeps the node/watchtower/api from functioning), warn (something that might want to be looked at but doesn't rise to an error), debug (for, well, debug purposes).

Rationale: This helps with parsing logs for alerting. Anything crit or error can be alerted on and wake the NO up. That should be the threshold for crit: Worth waking up at 2AM for, it's a "down" event. In the absence of crit, error can be handled the same way.