Coupon is not removed when invalid, no error message

When a coupon code is entered but invalid (eg. because the basket decorator does not fit requirements), no error message is displayed. The code just becomes part of the basket but does not have any effect on the basket value.

IMHO it would make more sense to explain the problem to the user like it happened in 2018.10 so the user can enter another coupon code or change his basket contents.

Same thing happens when the coupon code gets invalid because the basket content changed - the user may not even recognize the coupon is no longer valid. The invalid coupon code should be removed from the basket (IMHO) or at least the user should see a note explaning to problem.