JSCBC-142: Fixed issue where error objects lacked a message property.

Change-Id: I64785d09476fbb7a694960de6dac04f6492a4266
Reviewed-on: http://review.couchbase.org/41188
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>