I should quick note the main problem with trying to get the browser to show its default messages is that, to the best of my knowledge, browsers aren't always required to implement error pages. Most (if not all) have a page for when a domain doesn't resolve at all (404), but I have my doubts that they have their own built-in 403 Forbiddens since a 403 forbidden is returned by the webserver, and thus is an error page made -by- the web server.
About all I can suggest is what Dead-I suggested - make some generic plain-jane custom error pages and use those to override the x10 defaults.