Reads a single cron job run from the db.

def ReadCronJobRun(self, job_id, run_id):
    """Reads a single cron job run from the db."""
    for run in itervalues(self.cronjob_runs):
      if run.cron_job_id == job_id and run.run_id == run_id:
        return run
    raise db.UnknownCronJobRunError(
        "Run with job id %s and run id %s not found." % (job_id, run_id))