[STORY] Refactor 400/422 errors, standardize across repos

[ValerianClerc]

Currently, 422 errors are thrown in validators, and 400 errors in controllers. However, we might want to factor out 400 errors to a separate middleware, or include it in the existing validators.

Furthermore, in the user's microservice we check for existence of a conflicting username in the controller, with a 400 error. However, in the problems microservice, we check for the existence of problems/problemsets in the validators, returning 422 responses. These are similar validation checks, but they occur in different parts of the code and return different responses. This should be standardized/refactored.