How to read modelstate errors when returned by Json?

How can I display ModelState errors returned by JSON?

I want to do something like this:

 if (!ValidateLogOn(Name, currentPassword))
    {
        ModelState.AddModelError("_FORM", "Username or password is incorrect.");

        //Return a json object to the javascript
        return Json(new { ModelState });
    }

What must be my code in the view to read the ModelState errors and display them?

My actual code in the view to read the JSON values is as follows:

function createCategoryComplete(e) { 
    var obj = e.get_object(); 
    alert(obj.Values); 
}