How can I display modelstate errors returned by Json ?

I want to do somthing like that...

 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 is as follows to read the Json values:

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

}