title: Usage of AsyncListener#onError
body: <p>I don't understand when AsyncListener#onError method is called. 
  Javadoc doesn't help:</p>
  
  <blockquote>
    <p>Notifies this AsyncListener that an asynchronous operation has failed to complete.</p>
  </blockquote>
  
  <p>How could it fail? How can I reproduce this error?</p>
  
  <p><strong>UPDATE:</strong></p>
  
  <pre><code>// in HttpServlet doGet method
  AsyncContext asyncContext = req.startAsync();
  asyncContext.addListener(new AsyncListener() {
      // some code
      @Override
      public void onError(AsyncEvent event) {
          // when is it called?
      }
  });
  executeInSomeOtherThread(asyncContext);
  </code></pre>
  
  <p>What do I need to do in other thread to fail this async operation?</p>