Creating coupon code with duplicate code results in a scary TMI error

[michalrus]

[@Anna-ZZZ]
don’t fix the actual problem
insert validation that would run before creating a coupon (or a coupon prefix validation)
easy fix

/cc @alexanderfc

[michalrus]

Quite possibly solved by #2116, from what I understood about this issue. @bagratinho? :]

[michalrus]

It is solved, indeed, but with a scary error message:

Batch entry 0 insert into "coupon_codes" ("code","coupon_form_id","created_at") values ('ddd52',33,'2017-06-19 09:20:59.390000+00') was aborted: ERROR: duplicate key value violates unique constraint "coupon_codex" Detail: Key (lower(code::text))=(ddd52) already exists. Call getNextException to see other errors in the batch.

Test it here: https://appliance-10-240-0-25.foxcommerce.com/admin/promotions/1/coupons/new

The message must be better.