MB-6895 Deal with new mapreduce API

When a map call succeeds, the returned value is no longer
only a list of lists of KVs. An outter list element can
also be an error tuple now, which tells us there was an
error for a specific map function of a view.

Change-Id: If4a19c0d21e21756f749747841c07ec00b798d85
Reviewed-on: http://review.couchbase.org/24206
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>