[FIX] account: better message on user group error

The validation error introduced in 552da8dc can pop up in contexts where
it is not expected;
e.g. not in editing user rights but while upgrading a module.
Therefore we make it more explicit, giving the user a way to resolve the error.

closes odoo/odoo#32623

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>