During an import, account_invoice.py's _onchange_partner_id can return a
RedirectWarning exception, which wasn't properly handled by the load method.
Since it is an Exception, it was expected to have a name field.
We add it as a property so that they can be treated uniformly.
opw 1866965
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.
Christophe Simonis