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

src/applications/herald/engine/HeraldEngine.php

@@ -532,8 +532,12 @@ final class HeraldEngine extends Phobject {
     if ($caught) {
       $result_data = array(
         'exception.class' => get_class($caught),
-        'exception.message' => $ex->getMessage(),
+        'exception.message' => $caught->getMessage(),
       );
+      phlog(pht('An exception occurred executing Herald rule %s: "%s" Review '.
+        'the Herald transcripts and correct or disable the problematic rule.',
+        $rule->getMonogram(),
+        $caught->getMessage()));
     }
 
     $result = HeraldConditionResult::newFromResultCode($result_code)