Fix undefined variable warning in unit test field

Summary:
See D1295. $unit_messages may be undefined.

I'll see if I can improve the visibility of warnings, the red dot in DarkConsole
is easy to miss right now. See T734.

Test Plan: Loaded a revision with no unit failures, didn't receive a warning.

Reviewers: nh, btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1306