D6 version lacks 404/403 support if hook_js is missing

Hi,

With the move to the two-phase execution, it seems the handling of JS_NOT_FOUND and JS_ACCESS_DENIED was broken, since these are returned from the first phase, but checked against the return value of the second phase (which doesn't return them).

It also seems like JS_SITE_OFFLINE isn't ever returned?

I've attached a patch that fixes JS_NOT_FOUND and JS_ACCESS_DENIED, although dropping to a full bootstrap to 404 seems heavy for something that's meant to be lightweight. On our setup the memory limit is lower (24MB) for js.php than index.php, so drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL) actually crashes with a memory error in our case. Still, should be better for most people.