Handle 404s and mismatching resource types

[Treora]

For example, we currently happily inline an html 404 page as if it was the desired resource, producing e.g. <img src="data:text/html;base64,......">. We could consider alternatives, such as replacing such URLs with about:invalid.

[Treora]

I just noticed that when fetching a stylesheet results in an html error page, styles declared inside that page (inside a <style> tag) may actually be interpreted and applied by the browser (observed in Firefox and Chromium). Reminds of #17 (even though this does not corrupt the HTML itself, it does corrupt the applied styles).

This needs fixing.