add more information to isValidDescriptor invariant error

Just a minor tweak to tell you what's being returned (I had a problem where an array was being returned earlier today).  


This ternary is completely to work around a bug in PhantomJS's implementation of Object::toString which gives DOMWindow for some reason when called on `undefined` or `null` and caused the tests to fail.  V8, SM, and JSC all work correctly for all values (per spec).

```js
        renderedComponent === undefined 
          ? 'Undefined' 
          : Object.prototype.toString.call(renderedComponent).slice(8, -1),
```