Rename ValidationFailedMessage to BuildFailureMessage

ValidationFailedMessage is no longer in validation_pool and is used by
all builders. Rename it to something more meaningful.

BUG=chromium:371004
TEST=`cbuildbot/run_tests`

Change-Id: I238185e7acd23e233276ffff67deb1fc75d50f7f
Reviewed-on: https://chromium-review.googlesource.com/202918
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
7 files changed