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/server.py

@@ -220,6 +220,10 @@ class Server:
             self._restart = True
             self.interface.shutdown()
 
+    def shutdown(self):
+        self._restart = False
+        self.interface.shutdown()
+
     def _processCfg(self):
         errors = []
         opts = dict(self.node_cfg)