Errors instead of panics on closed delivery chan

When receiving from a closed delivery chan, a zero value Delivery could
be returned.  Calling Delivery.Ack() would panic without a clear reason
why.  Instead of panicing return an err indicating that the Delivery is
not initialized.

When ranging over the delivery chan, this error would never be returned.
fix import path "code.google.com/p/go.net/context" => "golang.org/x/net/context" in examples