[Dillo-dev] experimental patch: image error status as alt text

You know how for image redirections and 404s, nothing at all happens? User's mind: "I didn't get the image. Dillo must be broken." (or the address can't resolve, or content-type doesn't match...) So I tried making something recently that shows, e.g., [404] as the alt text. In this form it's probably wrong in a design sense because cache probably isn't allowed to call image, but maybe it'll prompt some discussion... (and maybe it'd be better to pull the status out of the header whenever needed rather than adding more junk to the cache entry.)