scanner: Return error for currently unsupported inclusion

Currently inclusion=low is only supported

Change-Id: I15449d772b73b00c20c16781f9f618653ee68759
Reviewed-on: http://review.couchbase.org/43555
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>