validateCouponCode does not fail promotions that do not pass the check method

[mschipperheyn]

Describe the bug
I have a custom promotion that is according to validateCouponCode valid and therefore applied. However, the check method returns false. But since the the coupon is considered valid it's applied with a value of zero and shows up in the ui. That doesn't make sense to me.

Expected behavior
Coupons that fail the check method (return false) should return a CouponInvalidError so you can process the ui accordingly.

Environment (please complete the following information):

• @vendure/core version: 3.0.8
• Nodejs version
• Database (mysql/postgres etc):