[fixes #330] “works” in Nashorn

* also fix `RSVP.on(‘error’,` on vertex

Please note: the test suite itself does not yet run in Nashorn so only some manual testing “proved” it would work. I have relatively good confidence in this, but at some point soon it would be better to get the test suite running on Nashorn. I do suspect it will likely be a sizable investment.

Conflicts:
	lib/rsvp/promise.js