Provide exception class to indicate internal SDK errors

Change-Id: Idd8c0cf06f7a2ea64250c9175457aceb6dd0d879
Reviewed-on: http://review.couchbase.org/26396
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>