Warn when trying to publish without starting publisher

[t-persson]

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