Add message for a combination of variations that's not available.

Hello,
A customer of mine wants to display a message when a variant is not availble of a product.
Some combinations of variants are not practical (like a table-top of 1 meter and a table-frame of 2 meters), so she selects them out. But they can still be ordered.
This patch adds a message to the frontend of the website.
It will also display the message if only half of the variations are selected.

I would also prefer to make the button un-clickable, but that might be a bit harder to do.

Please reply if this patch is acceptable.