Check if coupon availaible with discount condition formula

[fhamon]

The problem

Craft's commerce does not return an error when a user wants to add an existing coupon code to the cart via the action commerce/cart/update-cart and the order condition formula did not pass.

The solution

I added this simple check based on your code style and existing code to validate and send an error when the coupon is not valid because of the condition formula.

Please let me know if this PR is problematic, I am ready to change the code or adapt to your guidelines.

Thanks for Craft and thanks for the commerce plugin. Much love from Montreal ❤

[lukeholder]

Thanks for that!

[lukeholder]

Changelog entry: 3af212e