plugin: Make unannounced notification topics no longer fatal

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.

tests/plugins/custom_notifications.py

@@ -17,5 +17,19 @@ def emit(plugin):
     plugin.notify("custom", "Hello world")
 
 
+@plugin.method("faulty-emit")
+def faulty_emit(plugin):
+    """Emit a simple string notification to topic "custom"
+    """
+    plugin.notify("ididntannouncethis", "Hello world")
+
+
+@plugin.subscribe("ididntannouncethis")
+def on_faulty_emit(origin, payload, **kwargs):
+    """We should never receive this as it gets dropped.
+    """
+    plugin.log("Got the ididntannouncethis event")
+
+
 plugin.add_notification_topic("custom")
 plugin.run()