Reads a single cron job run from the db.

def ReadCronJobRun(self, job_id, run_id, cursor=None):
    """Reads a single cron job run from the db."""
    query = ("SELECT run, UNIX_TIMESTAMP(write_time) FROM cron_job_runs "
             "WHERE job_id = %s AND run_id = %s")
    num_runs = cursor.execute(
        query, [job_id, db_utils.CronJobRunIDToInt(run_id)])
    if num_runs == 0:
      raise db.UnknownCronJobRunError(
          "Run with job id %s and run id %s not found." % (job_id, run_id))

    return self._CronJobRunFromRow(cursor.fetchall()[0])