ofproto: Report nonexistent ports and queues as errors in queue stats.

ofproto: Report nonexistent ports and queues as errors in queue stats.

Until now, Open vSwitch has ignored missing ports and queues in most cases
in queue stats requests, simply returning an empty set of statistics.
It seems that it is better to report an error, so this commit does this.

Reported-by: Prabina Pattnaik <Prabina.Pattnaik@nechclst.in>
Signed-off-by: Ben Pfaff <blp@nicira.com>