[FIX] UserError: don't display None in Warning popup.

UserError uses a retrocompatibility function 'except_orm' which one can take a
second parameters. The default value of this second param is None, so if we
don't force an empty string, we will see a None on each raise UserError as last line.

odoo/exceptions.py

@@ -30,7 +30,7 @@ class except_orm(Exception):
 
 class UserError(except_orm):
     def __init__(self, msg):
-        super(UserError, self).__init__(msg)
+        super(UserError, self).__init__(msg, value='')
 
 
 # deprecated due to collision with builtins, kept for compatibility