I will add something simple to this comment. I am not aware of a property which can catch if you typed a URL and the URL cannot be reached, because the webserver is down or similar. What we can easily do is use the URL someone entered and perform a DNS lookup to see if the domain exists.
If it exists you will be able to access, if MWG cannot find any IP address for the host you will be redirected to Google.
Please note that this ruleset relies on a working and stable DNS and MWG needs to be able to perform DNS lookups.
Maybe it helps.
Thanks. That it`s what I have searched for.