MB-100: handle socket.error in data_helper methods

I suppose we need to handle socket.error as well as EOFError

http://qa.hq.northscale.net/view/2.0.1/job/ubuntu-64-2.0-new-rebalance-tests-P0/360/consoleFull

Change-Id: I05f64354029f381a25add0d24198a0cc6d77f9c6
Reviewed-on: http://review.couchbase.org/24663
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>