Shipping Estimator - Need error if not shipping in that region

Using 1.3.8.

I have shipping set up for US ONLY. If you try to check out and choose another country, you get an error message. That part works fine.

My question is regarding the shipping estimator. If someone tries to use that and is outside of our shipping area (US), I want the same error message. But instead nothing happens. No shipping is calculated.

How can I put the same error message.

Thank you,

Rather than restricting shipping to the US, why not remove all the countries other than the US so that non-US customers can't create an account?

I understand that is an option to remove the countries. I would rather have shipping estimator throw out an error. Right now, it just doesn't calculate anything. The space for the shipping costs is just blank.

Is there a way to program an error?

Thnx