Add two new sentinel errors. ErrNoStream and ErrNoConsumer

[actatum]

These sentinel errors will allow for better handling of managing streams and consumers.
Examples:

// Creating a stream when a stream does not exist.
si, err := js.StreamInfo(cfg.streamName)
if err != nil {
	if !errors.Is(err, nats.ErrNoStream) {
		log.Fatalf("js.StreamInfo(): %v", err)
	}
	_, err = js.AddStream(&nats.StreamConfig{
		Name:     "foo",
		Subjects: []string{"foo.>"},
	})
	if err != nil {
		log.Fatalf("js.AddStream(): %v", err)
	}
}

// Creating a consumer when one does not exist.
ci, err := js.ConsumerInfo(cfg.streamName, cfg.consumerName)
if err != nil {
	if !errors.Is(err, nats.ErrNoConsumer) {
		log.Fatalf("js.ConsumerInfo(): %v", err)
	}
	_, err = js.AddConsumer(cfg.streamName, &nats.ConsumerConfig{
		AckPolicy:     nats.AckExplicitPolicy,
		DeliverPolicy: nats.DeliverNewPolicy,
		Durable:       cfg.consumerName,
	})
	if err != nil {
		log.Fatalf("js.AddConsumer(): %v", err)
	}
}

[coveralls]

Coverage decreased (-0.1%) to 86.794% when pulling f830631 on actatum:sentinelErrors into 4b75fc5 on nats-io:master.

[wallyqs]

Thanks @actatum for making the changes! Could you squash the changes into a single commit?
Also I think would be good to change to ErrStreamNotFound and ErrConsumerNotFound so that they are similar to the errors from the server:

https://github.com/nats-io/nats-server/blob/77d54dabeb97c4f7e2d1cbe98e4cd33108e71560/server/errors.json#L351
https://github.com/nats-io/nats-server/blob/77d54dabeb97c4f7e2d1cbe98e4cd33108e71560/server/errors.json#L95

[actatum]

Thanks @actatum for making the changes! Could you squash the changes into a single commit?
Also I think would be good to change to ErrStreamNotFound and ErrConsumerNotFound so that they are similar to the errors from the server:

https://github.com/nats-io/nats-server/blob/77d54dabeb97c4f7e2d1cbe98e4cd33108e71560/server/errors.json#L351
https://github.com/nats-io/nats-server/blob/77d54dabeb97c4f7e2d1cbe98e4cd33108e71560/server/errors.json#L95

No problem, had a lot of fun doing this. Everything should now be squashed into a single commit!

[wallyqs]

LGTM :)