yui_combo should send 404s often

While reviewing MDL-41966, I detected (by forcing some invalid requests), that there are a bunch of situations where an textual error message is sent back to browser, surely with a wrong mime type and worse, with an "all-ok" 200.

This is about to review those places in yui_combo (especially all the "continue" statements there, or any other adding "error/wrong/not found" text to the $content, in order to 1) raise some "problem flag" and 2) based on that flag, return a 404 instead of current 200.

For your consideration, not critical but 200s should be sent for those unsatisfactory responses, IMO.