add more information to isValidDescriptor invariant error

[brigand]

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).

        renderedComponent === undefined 
          ? 'Undefined' 
          : Object.prototype.toString.call(renderedComponent).slice(8, -1),

[sebmarkbage]

I'm wondering if this needs to be extrapolated...? If it's needed here, isn't it also needed in every possible error message? Currently, the solution is to insert breakpoints so that you can inspect the exact value. It may be any other object which is also not valid.

Seems like this is trying to address arrays in particular, which is a confusing aspect of React. I think an isArray check with an error message linking or explaining why Arrays are not supported and potential workarounds would be better.