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/client/__init__.py

@@ -518,7 +518,7 @@ class SecopClient(ProxyClient):
         action, _, data = entry[2]  # pylint: disable=unpacking-non-sequence
         if action.startswith(ERRORPREFIX):
             errcls = self.error_map(data[0])
-            raise errcls('on SEC-Node: ' + data[1])
+            raise errcls(data[1])
         return entry[2]  # reply
 
     def request(self, action, ident=None, data=None):