JS: DeleteConsumer not found errors should be sentinel errors

[sloveridge]

Some of the errors reported from DeleteConsumer look to be just passed on from Nats Server and should probably be sentinel errors to be consistent with the rest of nats.go

Example:

package main

import (
	"fmt"
	"github.com/nats-io/nats.go"
)

func main() {
	nc, _ := nats.Connect(nats.DefaultURL)
	js, _ := nc.JetStream()
	if err := js.DeleteConsumer("TEST", "test"); err != nil {
		fmt.Println(err)
	}
	js.AddStream(&nats.StreamConfig{Name: "TEST"})
	if err := js.DeleteConsumer("TEST", "test"); err != nil {
		fmt.Println(err)
	}
	js.DeleteStream("TEST")
	nc.Close()
}

I would expect something like:

var (
    ErrStreamNotFound    = errors.New("nats: stream not found")
    ErrConsumerNotFound   = errors.New("nats: consumer not found")
)

[wallyqs]

Fixed via #760