Related: #20

Acked-by: Henning Leutz <leutz@pcsg.de>

Improve error handling for coupon code input.
Added a catch block to handle errors when adding a coupon code to the basket.
The error message is now displayed directly under the input field, providing a better user experience.

Related: #20

Modified 'var' declarations to 'const' in CouponCodeInput.js file for better ES6 compliance. This
refactoring helps increase code readability and ensures variables are block-scoped, reducing the
risk of accidental variable reassignment or mutation.

Related: #17