title: httpErrors defaultResponseMode ExecuteURL not working
body: <p>I have this in my web.config:</p>
  
  <pre><code>&lt;httpErrors errorMode="Custom" defaultResponseMode="ExecuteURL"&gt;
    &lt;remove statusCode="404" /&gt;
    &lt;error statusCode="404" path="/Error/NotFound" /&gt; 
  &lt;/httpErrors&gt;
  </code></pre>
  
  <p>And when I browse for this URL (which does not exist): <code>http://localhost:8605/a.png</code> it does not execute my <code>NotFound</code> action in my <code>Error</code> controller. Why?</p>
  
  <p>If I explicitely add <code>responseMode="ExecuteURL"</code> to the <code>error</code> element then it works.</p>
  
  <p>Like so: <code>&lt;error statusCode="404" path="/Error/NotFound responseMode="ExecuteURL" /&gt;</code></p>
  
  <p>Why can't I just set that attribute globally in the <code>httpErrors</code> element?</p>