Since plugins will start sending them soon, and they are likely to get
it wrong sometimes, be a bit more lenient, warn them in the logs
instead and then make sure it doesn't accidentally work anyway.
We will eventually start emitting and dispatching custom notifications
from plugins just like we dispatch internal notifications. In order to
get reasonable error messages we need to make sure that the topics
plugins are asking for were correctly registered. When doing this we
don't really care about whether the plugin that registered the
notification is still alive or not (it might have died, but
subscribers should stay up and running), so we keep a list of all
topics attached to the `struct plugins` which gathers global plugin
information.
Christian Decker