Fix dotest.py test runner exit code to return non-zero on failure/error.

This addresses this bug:
http://www.llvm.org/bugs/show_bug.cgi?id=21267

Which has been broken since svn r215256 on Aug 8 2014.

DO NOT REVERT THIS COMMIT EVEN IF IT CREATES TEST FAILURES.

The test failures are a result of accumulation of hidden failures
that were masked by the bug this change fixes.  Most of our test
runners as part of build testing rely on dotest.py returning non-zero to indicate
some kind of errant test run.  Thus, we have been flying blind
since Aug 8 2014.

llvm-svn: 219689