By default, Data Services don't return information on what's wrong when you try to execute some code, for example an update, against it. This is understandable: if you open up a data service on the web, you shouldn't be returning all important information about entities in the underlying model.
However, when developing a Data Service, it might be handy to get a little more information than just "An error occurred while processing this request". Here's how to achieve this...