improve error handling on callbacks

errors in callback functions should be reported to log,
but not stop the callback chain

Change-Id: I4fc509b7121960ebe59e1ad4f4b4746dfb4d5ba3
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30950
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

frappy/protocol/router.py

@@ -122,8 +122,7 @@ class Router(frappy.protocol.dispatcher.Dispatcher):
                             self.node_by_module[module] = node
                         self.nodes.append(node)
                         self.restart()
-                        return frappy.client.UNREGISTER
-                    return None
+                        raise frappy.client.UnregisterCallback()
 
                 node.register_callback(None, nodeStateChange)
                 logger.warning('can not connect to node %r', node.nodename)