Backend : Update endpoint /api/favorites/add with error handlers

[FlorentinaPetica]

Description

I updated the end point /api/favorites/add with error handlers

Fixes # 193

Please provide a short summary how your changes can be tested?

npm run dev check swagger in api/documentation make a post request with a id value of string or smaller that 1 and get a 400 error. If non existent id you get a 500 error.

Checklist

• [x] I have performed a self-review of my own code
• [x] 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
• [x] I have commented my code, particularly in hard-to-understand areas, if you code was simple enough mark the box anyway
• [x] I have made corresponding changes to the documentation, if you code was simple enough mark the box anyway
• [x] This PR is ready to be merged and not breaking any other functionality