title: Unable to pinpoint issue with GCP Composer (Airflow) DAG task failure
body: <p>I am new at using Apache Airflow.
  Some operators of my dag have a failed status. I am trying to understand the origin of the error. </p>
  
  <p>Here are the details of the problem:
  My dag is pretty big, and certain parts of it are composed of sub-dags.
  What I notice in the Composer UI, is that the Subdags that failed, all did in a <code>task_id</code> named <code>download_file</code> that uses <code>XCom</code> with a <code>GoogleCloudStorageDownloadOperator</code>.</p>
  
  <pre><code>
  &gt;&gt; GoogleCloudStorageDownloadOperator(
      task_id='download_file',
      bucket="sftp_sef",
      object="{{task_instance.xcom_pull(task_ids='find_file') | first }}",
      filename="/home/airflow/gcs/data/zips/{{{{ds_nodash}}}}_{0}.zip".format(table)
  )
  </code></pre>
  
  <p>The logs in the said Subdag do not show anything useful. </p>
  
  <p>LOG :  </p>
  
  <blockquote>
    <p>[2020-04-07 15:19:25,618] {models.py:1359} INFO - Dependencies all met
    for  [2020-04-07 15:19:25,660]
    {models.py:1359} INFO - Dependencies all met for  [2020-04-07 15:19:25,660]
    {models.py:1577} INFO -</p>
    
    <h2>------------------------------------------------------------------------------- Starting attempt 10 of 1</h2>
    
    <p>[2020-04-07 15:19:25,685] {models.py:1599} INFO - Executing
     on
    2020-04-06T11:44:31+00:00 [2020-04-07 15:19:25,685]
    {base_task_runner.py:118} INFO - Running: ['bash', '-c', 'airflow run
    datamart_integration.consentement_email download_file
    2020-04-06T11:44:31+00:00 --job_id 156313 --pool integration --raw -sd
    DAGS_FOLDER/datamart/datamart_integration.py --cfg_path
    /tmp/tmpacazgnve']</p>
  </blockquote>
  
  <p>I am not sure if there is somewhere I am not checking... Here are my questions :</p>
  
  <ol>
  <li>How do I debug errors in my Composer DAGs in general</li>
  <li>Is it a good idea to create a local airflow environment to run &amp;
  debug my dags locally?</li>
  <li>How do I verify if there are errors in XCom?</li>
  </ol>