Passing Exceptions to an error screen in ASP.net/C#

<customErrors mode="RemoteOnly" defaultRedirect="/error.html">
      <error statusCode="403" redirect="/accessdenied.html" />
      <error statusCode="404" redirect="/pagenotfound.html" />
   </customErrors>

Server.GetLastError();

protected void Application_Error(object sender, EventArgs e)
    {
        Exception ex = Server.GetLastError();
        this.Session[CacheProvider.ToCacheKey(CacheKeys.LastError)] = ex;
    }