title: Yii2 : How to Customize Error Pages like 404 and 503
body: <p>I have following config of errorHandler</p>
  
  <pre><code>'errorHandler' =&gt; [
      'errorAction' =&gt; 'page/error',
  ],
  </code></pre>
  
  <p>In the Controller page, in the Action error I want to check, that I got 404 error "page not found"?</p>
  
  <p>How can I check it?</p>