Story:
WHEN I request GET /entities/items/{item_id}
AND provide the _fields parameter including the value which is not allowed
THEN I get 400 HTTP response
AND it contains a invalid-field code and a message "Not a valid field: {field}"
AND a HTTP header Content-Language set to en

Tasks:

• use the validator to return a "failure" GetItemResult (with isSuccessful() == false)
• the RouteHandler will create an error response (set http status, code, message) based on the GetItemResult