More meaningful message when trying to ack message that was already acknowledged

[Berreek]

Feature Request

Currently when you ack the message that was already acknowledged you get following error:
nats: invalid jetstream publish response

Recently I got quite a lot of this in our application and it turned out that we performed double ack'ing since we did not have ManualAck option enabled. However we had to debug it trough to discover the root of this issue since it was not possible to understand it from the message above.

Proposed Change:

Change error message to something that will give you a actual context of the error. For example:
nats: message was already acknowledged

Don't mind doing a PR for it if you are fine with the change.

[wallyqs]

This sounds good to me, feel free to make the PR. Thanks.

[wallyqs]

Fixed via #862