Expose error constant for consumer name already in use

[mprimi]

Defect

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• Included nats.go version
• Included a [Minimal, Complete, and Verifiable example] (https://stackoverflow.com/help/mcve)

Versions of nats.go and the nats-server if one was involved:

nats.go v1.14.0

OS/Container environment:

N/A

Steps or code to reproduce the issue:

Try to add a consumer when a consumer with the same name exists

js.AddConsumer("some stream", consumerConfig)
js.AddConsumer("some stream", consumerConfig)

The error is expected, as the consumer exists!
However, one has to do manual string comparison because an error constant is not provided for this case.

Here's an example from this repo itself:

				(strings.Contains(cinfo.Error.Description, `consumer already exists`) ||
					strings.Contains(cinfo.Error.Description, `consumer name already in use`)) {

Expected result:

This is what I wish I could do:

  consumerInfo, err := js.AddConsumer(streamName, consumerConfig)
  if err == nats.ErrConsumerExists {
    // consumer exists - do what is appropriate
  }

Actual result:

This is what I'm forced to do (brittle and ugly):

  consumerInfo, err := js.AddConsumer(streamName, consumerConfig)
  if err != nil && fmt.Sprintf("%s", err) == "consumer name already in use" {
    // consumer exists - do what is appropriate
  }

[wallyqs]

Sounds good to me to add, we could have ErrConsumerNameAlreadyInUse similar to ErrStreamNameAlreadyInUse that already exists.

[ripienaar]

This client really should support the jetstream error codes :)