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/modules.py

@@ -642,7 +642,7 @@ class Writable(Readable):
     """basic writable module"""
 
     target = Parameter('target value of the module',
-                       default=0, readonly=False, datatype=FloatRange())
+                       default=0, readonly=False, datatype=FloatRange(unit='$'))
 
 
 class Drivable(Writable):