MB-100: handle socket.error in BatchedLoadDocumentsTask

http://qa.hq.northscale.net/view/2.0.1/job/centos-64-2.0-new-rebalance-mixed-cluster/49/console

Change-Id: Iafdc84635356b2b106096f05f6400c1eabaa9bc6
Reviewed-on: http://review.couchbase.org/24943
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>