clang_tidy: exit with an error code if clang-tidy fails

This also makes us print error output to stderr.

No functional change is intended.

BUG=chromium:960495
TEST=tryjob

Change-Id: Ib9f0a3cf2bc1fe67066c93ea128db27aec98935b
Reviewed-on: https://chromium-review.googlesource.com/1601197
Commit-Ready: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
1 file changed