How can I return custom error objects in Web API?

In my web API using the MVC 4 Web API framework, if there is an exception I'm throwing a new HttpResponseException:

if (!Int32.TryParse(id, out userId))
    throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Invalid id"));

This returns an object to the client: {"message":"Invalid id"}. I want to return a more detailed object:

{
 "status":-1,
 "substatus":3,
 "message":"Could not find user"
 }

How would I go about this? Is the best way to serialize my error object and set it in the response message? I've looked into ModelStateDictionary and came up with this, but it's not a clean output:

var msd = new ModelStateDictionary();
msd.AddModelError("status", "-1");
msd.AddModelError("substatus", "3");
msd.AddModelError("message", "invalid stuff");
throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, msd));