Error message when unregistering a connector from the Broker that was not registered

[sebplorenz]

You get an error message in the UI when unregistering a connector that is currently not registered at the Broker. I think this should not be an error message, because this situation can happen and can be foreseen. Instead, a message (not in red) should appear like: "The connector was not registered at the Broker."
After that the "Unregister" button must be a "Register" button again!!! If not, you don't have an option to re-register at the Broker.

Steps to reproduce:

1. Register connector at Broker.
2. On the Broker side remove the connector registration. By deleting the DB or removing the Broker from the Fuseki.
3. Unregister the connector from the Broker in the Connector UI.

Then the error message will be shown at the bottom of the screen.
Expected behaviour: Message (with a non-red background) at the bottom: "The connector was not registered at the Broker." The "Register" button must be active again.

[SebastianOpriel]

Thank you for reporting the issue.
Please do me a favor and reproduce the issue with DSC properties:

messaging.log.incoming=true
messaging.log.outgoing=true
```
and please provide me the message returned by the broker.