Summary:
For whatever reason, sc_testpilot default to --return-zero-on-failures, which
means that test failures are simply not reported properly to the signal box.
Fix that by explicitely passing --return-nonzero-on-failures

Reviewed By: fanzeyi

Differential Revision: D29509158

fbshipit-source-id: ef991f91df48e99769f96ffd8d7015cdf507c723