Re-throw exceptions immediately for completed generators.
Re-throw exceptions immediately for completed generators.

This happens to fix #210 indirectly, because it causes statements like
`return Promise.reject(error)` to throw, but there's a better way of
fixing that bug.

facebook/regenerator,RReverser/js-goto-transpiler,facebook/regenerator,liviuignat/regenerator,facebook/regenerator,rafaelstz/regenerator,RReverser/js-goto-transpiler,liviuignat/regenerator,rafaelstz/regenerator,juliocanares/regenerator,juliocanares/regenerator