title: Global Alert on any DAG Failure
body: <p>I currently have over 100 DAGs running in production. I am aware of how to add alerting with <code>on_failure_callback</code> and with operators triggered by upstream failures, but is there a way to configure Airflow itself to always send an email when a DAG fails without having to go through and update every one of my DAGs to alert on failure individually?</p>