Fixed SQL exception which occured on duplicate coupon code

When creating a coupon, if the entered code already exists, an exception is thrown and displayed to the user instead of a meaningful error message.

This PR fixes this.
on multi-line function call, first argument must be on a new line : https://scrutinizer-ci.com/g/thelia/thelia/inspections/595e980e-dd7b-4602-b94d-6862b314a1a6/issues/files/core/lib/Thelia/Form/CouponCreationForm.php?status=new&orderField=path&order=asc