Show an info view warning for ongoing or failed builds in Differential

Summary:
Fixes T10136. This reinforces ongoing or failed builds in the comment action area.

We already emit a similar message for unit test failures from arc unit. This should probably obsolete that, eventually.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10136

Differential Revision: https://secure.phabricator.com/D17140