Change the "You don't know where that is" response

Using the code below, if the player types “go some-unreachable-room”, the response is “You don’t know where that is.”

I’d like the response to be something else, e.g. “That’s not in this region.”

I’m new to Inform and can’t figure out how to change that response… I had a dig around in the documentation. 14.10 and 14.11 seem to be what I’m after but I’m still confused. I also had a good look in the Actions Index, but still couldn’t find that response. Perhaps I’m digging in the wrong place?

Any examples greatly appreciated.

Definition: A room is reachable if the map region of it is the map region of the location.

Teleporting is an action applying to one visible thing.
Understand "go [any known reachable room]" as teleporting.

Check Teleporting:
	if the player is in the location of the noun:
		say "You're already there.";
		stop the action;

Carry out Teleporting: move the player to the noun.

That response isn’t in the Standard Rules. Are you using an extension to define “known”?

I’m using an extension, only in the sense that I have split my code up into various extensions of my own which I include into the main file.

The code I’m using to define known and unknown is simply:

A room is either known or unknown.
A room is usually known.

The only other extension I’m using (i.e. not written by myself) is Basic Screen Effects by Emily Short.

Sorry, I missed it… I have a custom response to the “noun did not make sense in that context” error.

I can take it from here.