title: difference between email_on_failure and on failure callback
body: <p>I am very new to Airflow and did some researching but did not find any concrete difference .,and almost found that the job done by</p>
  <ul>
  <li>email_on_failure  by setting to true ie:<code>email_on_failure :True</code> and</li>
  <li><code>on_failure_callback=some_methods</code>- are almost same.
  -Can someone just mention the functional difference b/w these two?</li>
  </ul>