MP Product Shortcode - If none it does nothing?

Hi

I am using the market press product listing short code to display product listings of a certain tag.

If there are no products of that tag it simply does nothing.

I want it to display an error message or 'There are currently no products of this tag within this market press install'.

If you have a php function to determine 'if there are no products of tag=xyz' then that would be way better.

Thanks

Johnny