Description

Endpoint Products-on-discount will be able to handle the possible errors

Fixes # (195)

How to test?

Pull the branch Test for products on discount in the swagger.

[✔ ] I have performed a self-review of my code
[ ✔] I have followed the name conventions for CSS Classnames and filenames, Components names and filenames, Style filenames, if you are in doubt check the the project README.MD and here https://github.com/HackYourFuture-CPH/curriculum/blob/master/review/review-checklist.md
[ ✔] I have commented my code, particularly in hard-to-understand areas, if you code was simple enough mark the box anyway
[ ✔] I have made corresponding changes to the documentation, if you code was simple enough mark the box anyway
[ ] This PR is ready to be merged and not breaking any other functionality