(stable) Provide a more tailored error message when a Herald rule fails because of PCRE limits
Summary: Ref T13100. Since rules may begin failing for PRCE configuration reasons soon, provide a more complete explanation of possible causes in the UI.
Test Plan: Faked this, hit it via test console, saw explanation in web UI.
Maniphest Tasks: T13100
Differential Revision: https://secure.phabricator.com/D19178