Custom Errors for “App” folders? (ASP.NET)

void Application_Error(object sender, EventArgs e)
{
    //uncomment this to narrow down 'helpful' microsoft messages
    //HttpRequest request = ((HttpApplication)sender).Context.Request;


    Exception ex = Server.GetLastError();
    //ErrorManager is a custom error handling module
    ErrorManager.ProcessError(ex);
    Response.Redirect("~/error.aspx?error=" + HttpUtility.UrlEncode(ex.Message), true);
}