Coupon codes need unique value validation constraint.

let's try it

and there is a PR for this
https://github.com/drupalcommerce/commerce/pull/659

p.s. i also added a Validation test for Coupons there.

We can follow core's example and just extend the existing constraint, which means that we can customize the message without needing to write our own validator class.

Done.

Also replaced the "A coupon with code %value already exists. Enter a unique code." message with "The coupon code %code is already in use and must be unique." to make it match the SKU message. We can change both at the same time if we don't like the phrasing.

Waiting on travis before push: https://travis-ci.org/drupalcommerce/commerce/jobs/209730881