CQ: assert instead of re-running verifiers

This is not a valid state for CQ to be in,
and it makes CQ flood Rietveld with comments.

It does not happen under normal operation,
but has happened at least once. Exception
stack trace should help identify how CQ
went there and prevent it from misbehaving.

BUG=none

Review URL: https://codereview.chromium.org/41583004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/commit-queue@232201 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed