api – How to provide error details back to REST request?

In an ASP.NET MVC 4 REST API application, we have SQL errors which are occuring during the request.

However, since the requests are big, we are streaming the data back to the client, which means we have already returned the 200 OK response and headers.

So midway in the stream, something bad happens.
How can we indicate to the downstream client something happened?
With XML, we can at least just leave an incomplete file being returned.
With CSV though, I cannot come up with a clean way to indicate errors.

What’s the best practice for indicating an error occurred in the middle of a REST GET request? Google has somehow failed me on this one

Thank you