FileDownloader with ExternalResource, that does not exist

Hello,
I have a Filedownloader, that downloads external resource from another server new FileDownloader(new ExternalResource("http://.....")); It works fine, when resource exists. But, when resource does not exist I would show a notification. Unfortunately I haven’t found a method, where to do it as
handleConnectorRequest(VaadinRequest, VaadinResponse, String)
is not called in such case. What should I do to be able to show notification, when requested resource doesn’t exist?

Cheers
Agata