Log Herald rules with invalid keys via phlog()

Summary:
When Herald rules fail unexpectedly due to relying on a (now) invalid key, allow Phorge administrators to get aware by creating an entry in the error log via phlog() instead of crossing fingers that someone may from time to time get the idea to open and read Herald transcripts.

Refs T15885

Test Plan: See steps in T15885; check error log.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15885

Differential Revision: https://we.phorge.it/D25735