[PKT_SCHED]: GRED: Improve error handling and messages

Try to enqueue packets if we cannot associate it with a VQ, this
basically means that the default VQ has not been set up yet.

We must check if the VQ still exists while requeueing, the VQ
might have been changed between dequeue and the requeue of the
underlying qdisc.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>