Treat NoneType failure messages as infra failures

Normally, all failed builders should report a BuildFailureMessage. If
a builder reports a NoneType message, treat it as an infra failure.

BUG=chromium:381297
TEST=`cbuildbot/run_tests`

Change-Id: Ib334492c2d9e49cff4b9253255988bbe46110a43
Reviewed-on: https://chromium-review.googlesource.com/202836
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
3 files changed