QtWebkit: How to check HTTP status code?

I'm writing a thumbnail generator as per an example in the QtWebkit documentation. I would like to avoid screenshots of error pages such as 404 not found or 503 Internal server error.

However, the QWebPage::loadFinished() signal is always emitted with ok = true even when the page gives an HTTP error. Is there a way in QtWebkit to check the HTTP status code on a response?