Fix error codes in native index builder and index cleaner

Change-Id: I8dc0bb742ae51fb3112f038969ef00d81aae22ce
Reviewed-on: http://review.couchbase.org/31657
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>