If a {resource} doesn't exist, fall back to 404 URL

[Colonel_mortis]

At the moment, if a {resource} doesn't exist, you return an empty string. This means that the browser will try to use the html document as the resource, which obviously wouldn't work and may, in some browsers, lead to an extra HTTP request for that page, increasing server load. If instead you returned a URL that pointed to an image that displayed an error (or just something like a red square, so that it's visibly broken), at least in designer and dev modes, and which returned status code of 404, it would make it much easier to notice and debug.