[IMP] snailmail: Send an error when using non-A4 paperformat.

Since the provider only accepts A4 letters, prevent the user from using
non A4 formats by giving them an error when clicking the Send button.

closes odoo/odoo#105150

X-original-commit: e03fae1f
Signed-off-by: Florian Daloze (fda) <fda@odoo.com>