JSCBC-154: Corrected Error class not being attached in JS-Land.

Change-Id: I77377a9320a10a491f5b86a2b74f71589298f04c
Reviewed-on: http://review.couchbase.org/42371
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>