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));

I think this will do the trick:

Create a custom exception class for the business layer:

 public class MyException: Exception
 {
    public ResponseStatus Status { get; private set; }
    public ResponseSubStatus SubStatus { get; private set; }
    public new string Message { get; private set; }

    public MyException()
    {}

    public MyException(ResponseStatus status, ResponseSubStatus subStatus, string message)
    {
        Status = status;
        SubStatus = subStatus;
        Message = message;
    }
 }

Create a static method to generate a HttpError from an instance of MyException. I'm using reflection here so I can add properties to MyException and always have them returned w/o updating Create:

    public static HttpError Create<T>(MyException exception) where T:Exception
    {
        var properties = exception.GetType().GetProperties(BindingFlags.Instance 
                                                         | BindingFlags.Public 
                                                         | BindingFlags.DeclaredOnly);
        var error = new HttpError();
        foreach (var propertyInfo in properties)
        {
            error.Add(propertyInfo.Name, propertyInfo.GetValue(exception, null));
        }
        return error;
    }

I currently have a custom attribute for a general exception handler. All exceptions of type MyException will be handled here:

public class ExceptionHandlingAttribute : ExceptionFilterAttribute
{
    public override void OnException(HttpActionExecutedContext context)
    {
        var statusCode = HttpStatusCode.InternalServerError;

        if (context.Exception is MyException)
        {
            statusCode = HttpStatusCode.BadRequest;
            throw new HttpResponseException(context.Request.CreateErrorResponse(statusCode, HttpErrorHelper.Create(context.Exception)));
        }

        if (context.Exception is AuthenticationException)
            statusCode = HttpStatusCode.Forbidden;

        throw new HttpResponseException(context.Request.CreateErrorResponse(statusCode, context.Exception.Message));
    }
}

I'll play around with this a bit more and update as I find holes in this plan.