The thing is, you can determine that the site exists if you get a return code between 200 and 299, but you cannot determine that it does not exist.
If you get some other return code (even 404) it may only be a temporary error, or you may not be connected with the net, etc, etc, etc.
So, you can determine that it exists, but not that it doesn't exist. The only thing you can really say is that it is momentarily unavailable.