CBQE-0: get stats if _verify_all_buckets failed

Change-Id: I8d673d0ef653f0d64f851cba36af696c7ec4814b
Reviewed-on: http://review.couchbase.org/26901
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>