scanner: Return error to query if scannable snapshot is unavailable

Change-Id: I1d1bf86fa583f48f26aef232cf8d0b8a8210b103
Reviewed-on: http://review.couchbase.org/43544
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>