various small changes

- set default target unit to '$'
- shorten too verbose error message on client
- add shutdown method to server and dispatcher

Change-Id: Ib3a8b26bc31e988e45a3ff2efd734168d723d794
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/27095
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/protocol/dispatcher.py

@@ -83,6 +83,7 @@ class Dispatcher:
         self._subscriptions = {}
         self._lock = threading.RLock()
         self.restart = srv.restart
+        self.shutdown = srv.shutdown
 
     def broadcast_event(self, msg, reallyall=False):
         """broadcasts a msg to all active connections