Improved warning message when accepting diff with skipped lint or unit

Summary:
Some people find the current message stating "This diff has Lint/Unit Test
Problems" confusing if the unit tests or lint was skipped. This revision
clarifies those messages.

Test Plan:
Started to accept a revision with skipped lint and unit tests, and saw the new
message.

Reviewers: epriestley, btrahan, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1738