Most likely you would have to add your own URL handler (like in this post javafx 2 webview custom url handler, don't work relative url) and retrieve/detect 404 messages and conditions yourself.
This article (How to check if a URL exists or returns 404 with Java?) shows you how to detect a 404 condition.
In the handler you have 2 choices, either create a HEAD
request for a URL (and subsequently ignore HEAD
requests so you don't get yourself into an infinite loop) or masquerade the returned URLConnection after checking for a 404.
@Override
protected URLConnection openConnection(URL u) throws IOException {
// 1. Use Apache HTTPClient or something else to grab the url
// 2. Read the resultCode and report if it's a 404
// 3. Return an object that subclasses URLConnection
}