Fix an issue where we would try to release an unheld lock

Summary: Fixes T7484. If the lock failed, we'd still try to unlock it, which is incorrect.

Test Plan: Ran two bin/repository update X in different windows, got proper LockException instead of indirect symptomatic "not locked by this process" exception.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7484

Differential Revision: https://secure.phabricator.com/D12253