[IMP] l10n_ar: change message error

The error message now shows that a VAT tax must be added to that line
and that if it is already added, it may be misconfigured. This improves
the functionality since previously it showed that there should only be
one VAT tax and that could generate confusion in some cases

[CLA] add new members to Adhoc CLA

closes odoo/odoo#89089

Signed-off-by: Josse Colpaert <jco@odoo.com>