A handy usage pattern for autoninja.bat that I was not aware of is to go

    autoninja -C out\Default chrome && chrome

This will build chrome and then run it, but only run it if the build
succeeds. The addition of post_build_ninja_summary.py broke this by
losing the error code. This change fixes it by using black magic to
set an error code in the failure case.

Bug: chromium:787983
Change-Id: Ib87fd1799816e19d56de76e08e0f9688be903d80
Reviewed-on: https://chromium-review.googlesource.com/916705Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>