Fix ScriptError to always return string from __str__

Use super for proper handling of exceptions. msg property was added due
compatibility with the current code.

https://fedorahosted.org/freeipa/ticket/6294

Reviewed-By: Tomas Krizek <tkrizek@redhat.com>

ipapython/admintool.py

@@ -38,11 +38,12 @@ class ScriptError(Exception):
     def __init__(self, msg='', rval=1):
         if msg is None:
             msg = ''
-        self.msg = msg
+        super(ScriptError, self).__init__(msg)
         self.rval = rval
 
-    def __str__(self):
-        return self.msg
+    @property
+    def msg(self):
+        return str(self)
 
 
 class AdminTool(object):