(stable) Fix a commit hook issue where pushing dangerous changes would fatal before hitting the dragon bureaucrats

Summary: See https://discourse.phabricator-community.org/t/php-fatal-when-using-git-push-d/1317. The behavioral changes for Herald on initial import from D19265 could leave $all_updates undefined if we throw early enough.

Test Plan: Pushed a dangerous change, saw dragon bureaucrats again.

Differential Revision: https://secure.phabricator.com/D19297