can't bind to a validatedObservable as the root

i'm trying to do validation across an entire VM and have been running into problems somewhat like the guy @ http://stackoverflow.com/questions/13364788/knockout-validation-validatedobservable-group-error did. He found a solution to his problem @ http://jsfiddle.net/CGuW2/6/ but it seems flawed to me because he is doing roughly this:

``` javascript
var viewModel = {
num1: ko.observable("50").extend({ number: true, min: 0, max: 100 }),
num2: ko.observable("50").extend({ number: true, min: 0, max: 100 })
};

viewModel.isValidSum = ko.validatedObservable({
num1: viewModel.num1,
num2: viewModel.num2
}).extend({ mustEqual: 100 });

ko.applyBindings(viewModel);
```

Seemed weird that the VM had a property that was the VM only observed...and reading the answer at https://github.com/ericmbarnard/Knockout-Validation/issues/41 i thought something like this would be the "right" answer:

``` javascript
var viewModel = {
num1: ko.observable("50").extend({ number: true, min: 0, max: 100 }),
num2: ko.observable("50").extend({ number: true, min: 0, max: 100 })
};

var vm = ko.validatedObservable(viewModel).extend({ mustEqual: 101 });

ko.applyBindings(vm);
```

But when i use this with a tweaked View (see http://jsfiddle.net/drdamour/5B8s4/ ) the validation of the individual input boxes works, but the error property can't be bound to, although it is there, verified by a console.log.

Not being able to use a validatedObservable as a root VM seems like a bug that isn't intended. Is it intended that you can use extend on a validatedObservable to do VM level validation?

Reproduce the validatedObservable-as-root case from the issue and compare it with the linked jsFiddle examples. Trace how the root observable is exposed to bindings, then confirm that VM-level validation errors can be bound while individual input validation continues to work.

Written by the indexing model from the issue text.