feat(alerting): Add failure alerts to catch explicit failures during invocation.

Configure optional failure alerts, which explicitly detect a non-successful invocation, rather than looking at the absence of a successful invocation. In most cases the existing alert is sufficient, but the new alerting policy is useful for scripts which are invoked infrequently and therefore we cannot use the existing alert due to the monitoring window having to be less than 24 hours.

Closes #10 (closed)