Notify functions should check if the client is closed

[Zerpet]

rabbitmq-stream-go-client/pkg/raw/client.go

Lines 1289 to 1296 in b708687

	func (tc *Client) NotifyPublishError() <-chan *PublishError {
	tc.mu.Lock()
	defer tc.mu.Unlock()
	p := make(chan *PublishError, 1)
	tc.publishErrorCh = p
	return p
	}

These functions could check that the client is not closed, nor closing. We could change the signature to return a channel and an error, and return a specific error when the client is not open.

[Zerpet]

I'm undecided if it's needed, other than for correctness. I came up with this when I was thinking about the idempotency of raw.Client.Close() and Client.shutdown(). Given that all notify and shutdown function synchronise, this may not be needed at all.