Fix Composition Level of Components w/o Owners

This fixes a bug with components constructed with no owners, for example:

  // Both the <div> and <span> have no owners.
  React.renderComponent(<div><span /></div>, node);

They should have a composition level of 1 and their keys should be prefixed with 0 to indicate they were created without owners. However, they currently incorrectly get a composition level of 0 (which means that //their// children will have keys prefixed with 0, which is wrong).
Throw on Missing Elements

This changes React to throw when `ReactID.getNode()` fails to find a node. This method is used by two call sites:

 - Implements `ReactComponent#getDOMNode`. This method already throws if a component is not mounted, and //all mounted components should be able to find their rendered root nodes//.
 - Used by `ReactDOMIDOperations`. These call sites aleady assume that `getNode` returns a non-null. Currently, if the node is not found, this is the site that fatals (and the stack trace is much harder to debug).

The error message should make it //a lot// easier to debug unexpected DOM trees. In particular, this will help track down all the places where the browser inserts `<tbody>` unexpectedly.
.reactRoot[base10] -> .r[base36]

Just a bit of byte savings for server rendering. Props to @benjamn for the base36 idea (and for making this diff easy).

With a little work we could probably get rid of the .r as well.