BDD
WHEN I request GET /entities/items/{item_id} with a text which is not an item ID
THEN I get 400 HTTP response
AND it contains a invalid-item-id code and a message "Not a valid item ID: {item_id}"
AND a HTTP header Content-Language set to en

Task breakdown notes:

• create a Validator on the "use case level" that will check the item ID from the request and return a ValidatioResult object
• if the ValdationResult has violations, an error result (with isSuccessful() == false) will be returned from the GetItem use case
• the RouteHandler will create an error response (set http status, code, message) based on the GetItemResult