Improve delta recovery error message.

Change-Id: Ib20861bc4326e2ec66099ce3e3c46a77e91ebf2d
Reviewed-on: http://review.couchbase.org/40443
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>