gistfile1.txt

public HttpResponseMessage Something()
{
   if (someErrorCondition)
   {
       return new HttpResponseMessage<ErrorResponse>(new ErrorResponse("Blah error occured"), HttpStatusCode.500); // Whatever the code is
   }

   return new HttpResponseMessage<Result>(myResult);
}

public class ErrorResponse
{
    public ErrorResponse()
    {
        
    }

    public ErrorResponse(string error)
    {
        Error = error;
    }

    public string Error {get;set;}
}

//If client sends accept: application/xml they will get something like

<ErrorResponse>
    <Error>my error text>
</ErrorResponse>


//If client sends accept: application/json they will get something like

{ "_error": "my error msg" }