Use better deactivate error message in products
In a "clean" tryton installation: 1.- Install product module and its dependencies. 2.- Create a new product.template with at least one product.product 3.- Try to deactivate the product template without deactivate the product variants 4.- Try to save, this return the message: 'The values of "Variants" are not valid.' 5.- This message it is not clear for the user, we need to use a validate and return a message like: "You need to first deactivate the product variants to deactivate a product template"