Update backend to make sure API returns correct HTTP status codes

[zkwsk]

Go through the example endpoint setup and ensure that 404 is returned if you try to request a resource that's not there and otherwise give a 5xx code.

I don't think we currently have backend authentication backed into the boilerplate, but if we do, we should also make sure that we return 401 codes on unauthorized requests and so on.

To be honest, I did not check what we actually have in the codebase I just know that we've had issues on most projects with this but I don't remember how much was in the boilerplate and how much was written by students. If it's not in the boilerplate we should add a guide explaining to the students how to set up the status codes correctly.