Send alert when remove-ineligible-domains cron job fails

The cloud console (e.g. https://console.cloud.google.com/cloudscheduler?project=hstspreload) only shows the status of the last run of the cron job. If it fails, we should get notified somehow of the failure so that we have a record of how often it fails and so we can retry if needed. (It's possible but cumbersome to look through stderr in the logs explorer to find successful vs unsuccessful runs.)