Changed the postponed unit tests warning message

Summary:
Postponed unit tests are not unit tests with problems. The results
just haven't arrived yet.

Test Plan: Tested accepting a diff with unit status 1, 3, 5 (ok, errors,
postponed)

Reviewers: jungejason, epriestley

Reviewed By: epriestley

CC: aran, epriestley, tuomaspelkonen

Differential Revision: 969