Inconsistency in behavior of Service::Customer#fetch_by_id between QBD and QBO

[mcmire]

When requesting a customer by id and it is not found, Windows will return nil, but Online will throw an error. We should either throw an error or return nil in both cases. I would personally return nil -- there is no reason to make calling code blow up, especially since this never occurs when returning a list a customers and the list happens to be empty.