signer scripts: return an error when loading configs fail

If we are given a config file that doesn't exist, return an error rather
ignoring it.  This way we don't accidentally give images a pass when we
didn't properly test them.

BUG=None
TEST=`./security_test_image --board=x86-alex --baselines=/` now shows 3 failures rather than all pass
BRANCH=None

Change-Id: I9d130db05befaeac8fcca921f0e43f47c2461f9f
Reviewed-on: https://gerrit.chromium.org/gerrit/34795
Reviewed-by: Ryan Cui <rcui@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
3 files changed