LibWeb: Don't call ResourceLoader error callback on 4xx status code

A website with a 4xx status code is still a valid website, we should not
artificially hide it. In fact, many websites implement custom 404 error
pages for example, often containing search functionality or links back
to the homepage.
This might have implications regarding the loading of stylesheets where
the request 404s, but since we were not handling 5xx status codes as
errors either, I think that's fine for now (but might need additional
work later). Worst case, the parser rejects to load some error page HTML
as CSS :^)

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