CBQE-0: handle ServerUnavailableException in delete_all_buckets_or_assert

Change-Id: I28c581f9c19ba20ae23a94b7f94b7a618a2435c0
Reviewed-on: http://review.couchbase.org/26906
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>