[REF] l10n_ec: Message validation for VAT

- We refactored the VAT validation from a validation error to a warning that is stored in l10n_ec_vat_error
- Remove the "-" and "EC" from the base_vat in the EC validation/example
- Only validate the length in the base_vat
- Add compute field with the warning message in the l10n_ec

closes odoo/odoo#121099

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