Add ErrKeyExists that is compatible with errors.Is on kv.Create

[wallyqs]

Follow up from (#1135)
This changes the returned error on kv.Create now to be wrapping the APIError and show error information relevant to the kv.Create API as well:

nats: wrong last sequence: 1: key exists

instead of:

nats: wrong last sequence: 1

The wrapped error makes it possible to make checks like this as well:

_, err = kv.Create("key", []byte("1"))
if !errors.Is(err, ErrKeyExists) { ... }

And also still possible to get the original APIError if needed:

_, err = kv.Create("key", []byte("1"))
var aerr *APIError
if errors.As(err, &aerr) { ... }

Signed-off-by: Waldemar Quevedo wally@nats.io

[coveralls]

Coverage decreased (-0.02%) to 85.798% when pulling 5e12137 on js-kv-err into 5044b6e on main.

[bruth]

This looks solid to me and retains backwards compatibility except for the, hopefully rare/no longer used practice of, doing if err.Error() == "..." { ... }.

[piotrpio]

LGTM