various fixes

- nodestatechange callback must appear after the online attribute
  is changed
- IntRange: move range validation outside of try except
- fixes on some error names
- well defined error message 'no such class' in secop.lib.get_class
- try again 4 times when starting Tcp Server on EADDRINUSE
- fix error handling

Change-Id: I4eee9b79ea8173936b9f5193b87e006ac8fca827
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/26171
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/lib/__init__.py

@@ -105,7 +105,10 @@ def get_class(spec):
     else:
         # rarely needed by now....
         module = importlib.import_module('secop.' + modname)
-    return getattr(module, classname)
+    try:
+        return getattr(module, classname)
+    except AttributeError:
+        raise AttributeError('no such class') from None
 
 
 def mkthread(func, *args, **kwds):