title: How do I force a task on airflow to fail?
body: <p>I have a python callable <code>process_csv_entries</code> that processes csv file entries. I want my task to complete successfully only if all entries were processed successfully. Task should fail otherwise</p>
  
  <pre><code>def process_csv_entries(csv_file):
      # Boolean 
      file_completely_parsed = &lt;call_to_module_to_parse_csv&gt;
      return not file_completely_parsed
  
  CSV_FILE=&lt;Sets path to csv file&gt;
  t1 = PythonOperator(dag=dag,
                        task_id='parse_csv_completely',
                        python_operator=process_csv_entries,
                        op_args=[CSV_FILE])
  </code></pre>
  
  <p>t1 seems to complete successfully irrespective of returned value.
  How do I force PythonOperator task to fail?</p>