When username already existing is used for registration, a 400 is returned with the message "rejected" change this to something more meaningful like: "User with username x already exists".
Endpoint: POST users/ -> body {"username": "initialUser", "email": "test@test.com", "password": "password"}
Endpoint: POST users/ -> body {"username": "initialUser", "email": "test@test.com", "password": "password"}