title: HttpError will not show custom error pages
body: <p>I've got this in the web.config:</p>
  
  <pre><code>&lt;httpErrors errorMode="Custom"&gt;
    &lt;remove statusCode="404" subStatusCode="-1" /&gt;
    &lt;remove statusCode="500" subStatusCode="-1" /&gt;
    &lt;error statusCode="404" prefixLanguageFilePath="" path="/Error/NotFound.aspx" responseMode="Redirect" /&gt;
    &lt;error statusCode="500" prefixLanguageFilePath="" path="/Error/ServerError.aspx" responseMode="Redirect" /&gt;
  &lt;/httpErrors&gt;
  </code></pre>
  
  <p>But IIS still shows the built in error page.</p>
  
  <p>Any ideas?</p>