MB-100: better logging on delete_all_buckets_or_assert

Change-Id: Iffcefb7455f492f950d42f5d8ee26374d581e3b4
Reviewed-on: http://review.couchbase.org/24656
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>