no message in expection in future version of python

mongoengine/base.py

@@ -10,7 +10,12 @@ def get_document(name):
 
 
 class ValidationError(Exception):
-    pass
+    def _get_message(self, message): 
+        return self._message
+    def _set_message(self, message): 
+        self._message = message
+    message = property(_get_message, _set_message)
+
 
 
 class BaseField(object):