Provide a more structured result log for Herald conditions

Summary:
Ref T13586. Currently, Herald condition logs encode "pass" or "fail" robustly, "forbidden" through a sort of awkward side channel, and can not properly encode "invalid" or "exception" outcomes.

Structure the condition log so results are represented unambiguously and all possible outcomes (pass, fail, forbidden, invalid, exception) are clearly encoded.

Test Plan:

Maniphest Tasks: T13586

Differential Revision: https://secure.phabricator.com/D21563