Fix fatal when looking at an unknown-config error.

Summary:
Going to /config/issue/config.unknown.phabricator.setup/ fataled with

Call to a member function getLocked() on a non-object

Test Plan: Went to /config/issue/config.unknown.phabricator.setup/ and saw the page render.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4633