Change jetstream::Error's Display message

[thinety]

The current Display message for jetstream::Error is a little confusing. For example, this piece of code:

#[tokio::main]
async fn main() -> Result<(), async_nats::Error> {
    let client = async_nats::connect("localhost:4222").await?;
    let jetstream = async_nats::jetstream::new(client);
    if let Err(err) = jetstream.get_object_store("nonexistent").await {
        eprintln!("{}", err);
    }
    Ok(())
}

prints

failed to get Object Store: jetstream error: 404 (code stream not found, error code 10059)

After this change, the message is:

failed to get Object Store: jetstream error: stream not found (code 404, error code 10059)

[Jarema]

This is a nice improvement, thanks!

LGTM!