CBQE-2340: added support to ignore expected memcached errors during load data

Change-Id: I233b36c5e2f3ff6f2eb71f9f3c55d20b75a5d86b
Reviewed-on: http://review.couchbase.org/40303
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>