setting the error flag in the OCUnitIOSAppTestRunner

This is a clean-up of https://github.com/facebook/xctool/pull/230.

For this version I simply renamed gotUncaughtSignal to gotError, since it is used to mark things as 'errors'. It also turns out there are a couple cases where 'gotError' is set, but 'error' isn't set, so I think the boolean was probably originally added for a reason.