Added a big warning if reviewer is about to accept a diff with lint or unit
errors.

Summary:
Make sure reviewers know what they are doing.

Test Plan:
Tested with different diffs that had lint and unit problems.

Reviewed By: epriestley
Reviewers: epriestley, jungejason
CC: grglr, aran, epriestley, tuomaspelkonen
Differential Revision: 432