don't explode if a query no longer has failures

lesson from week #3 of elastic recheck, people actually fix the
bugs that we identify. Which is awesome!

However, check_success.py never accounted for the crazy idea that
one of our bugs might no longer have failure results in elastic
search. Change the way we compute failures so we can survive fixed
bugs.

Change-Id: I3fbbfc1060721dc11bdc5e56ba0d246ed95e70af