How about catch it at EndRequest event?
protected void Application_EndRequest(object sender, EventArgs e)
{
HttpRequest request = HttpContext.Current.Request;
HttpResponse response = HttpContext.Current.Response;
if ((request.HttpMethod == "POST") &&
(response.StatusCode == 404 && response.SubStatusCode == 13))
{
// Clear the response header but do not clear errors and
// transfer back to requesting page to handle error
response.ClearHeaders();
HttpContext.Current.Server.Transfer(request.AppRelativeCurrentExecutionFilePath);
}
}