Story:
WHEN I request GET /entities/items/{item_id} with ID of item that does not exist
THEN I get 404 HTTP response
AND it contains a item-not-found code and a message "Could not find an item with the ID {item_id}"
AND a HTTP header Content-Language set to en

Tasks:

• return null from WikibaseEntityLookupItemRevisionRetriever in case no entity revision can be found for a given ID
• if item revision is null, then a "failure" GetItemResult (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