Throw error if configurable product wants to use non existing configurable attribute for child product

[pquerner]

Product A is a configurable product. Product B is a simple product.

A wants product B as its child. A has set it wants to have attribute "example" as configurable attribute. Product B doesnt have a value for the "example" attribute.

Therefore the products are both linked in the super_link table, but are not visible in the backend.

Reason: the missing attribute.

I'd like to throw an error somewhere if that happens, so we are not in the dark of "why its not working".