Warn when trying to publish without starting publisher

### Description

When trying to publish a message when the publisher is closed, you get no feedback.
It's better to send a warning or just crash when this happens.

### Motivation

Information is king

### Exemplification

Trying to debug why messages did not arrive at subscriber takes up a lot of time before realizing that you hadn't started the publisher.

### Benefits

Information is king.

### Possible Drawbacks

none