Where is statusDescription When .NET MVC Return HttpNotFound

The code like this

if (id == null)
{
    return HttpNotFound("Error Id");
}
//// do something ...
if (entity == null)
{
    return HttpNotFound("Null Entity");
}
//// do something ...
if (somethingError())
{
    return HttpNotFound("Something Error");
}

If I Request it use browser ,

I will got a 404 Page which I set before .

But where is the Error Message(statusDescription) ?

I can not find it on the Header or Raw.