Add UI help text re: case-insensitivity for coupon code matching

@ozin: The second point from the issue was ignored.
Let's update the error message to be:

Coupon code matching is case-insensitive, and codes must be unique. %value cannot be used because it matches an existing code.

ofc we need to update the tests accordingly.

Getting the existing code would require defining a custom constraint validator and considering the logic baked into that validator, let's pick an easier route and let's just update the validation message.

Automatically closed - issue fixed for 2 weeks with no activity.