Log decoded KV list on reduce failure

So that we don't need to decode it when debugging failures.
This was not the case before support for termless format
storage was added.

Change-Id: I38019caf5235c234441c42ab2650540cf201deef
Reviewed-on: http://review.couchbase.org/25361
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>