How can I simulate a 'down' website?

Edit your hosts file to redefine the host you're trying to connect to. You can do 127.0.0.2 (Or anything unreachable).

You can also do a test with 0.0.0.0 - that returns a different error (Invalid IP). There may be some benefit to testing for that too.

Your ISP is redirecting for a DNS lookup failure, but anything resolved by your hosts file short-circuits that.

http://localhost

Assuming you do not have a server running locally...

In order to test how your scripts deal with different responses I'd rather use a special server that generates different HTTP codes. One of them is httpstat.us

In your case, the correct http status code is 503 (Service Unavailable).

The 503 status code indicates that the server is currently unable to handle the request due to a temporary overload or scheduled maintenance

which will likely be alleviated after some delay.

For more information on http status codes I suggest reading This IETS Documentation