Don't unpublish without or with invalid API key

[dkastl]

When a subscription is unpublished, this currently also works on the Redmine side when the API key is not provided or invalid. So in this case the subscription does not get removed on the Orion server side but the subscription ID is removed on the plugin side.

This behaviour might be useful, but the user should be at least informed that the unpublishing didn't work at the Context Broker.