СBQE-1119: handle diff errors in testClusterInitNegative

Change-Id: Idcad40e3a388f33444582e62a0ccd828d1a5acf2
Reviewed-on: http://review.couchbase.org/27287
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>