[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#105058

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

addons/snailmail/models/snailmail_letter.py

@@ -127,6 +127,9 @@ class SnailmailLetter(models.Model):
             else:
                 report_name = 'Document'
             filename = "%s.%s" % (report_name, "pdf")
+            paperformat = report.get_paperformat()
+            if (paperformat.format == 'custom' and paperformat.page_width != 210 and paperformat.page_height != 297) or paperformat.format != 'A4':
+                raise UserError(_("Please use an A4 Paper format."))
             if not self.cover:
                 raise UserError(_("Snailmails without covers are no longer supported in Odoo 13.\nPlease enable the 'Add a Cover Page' option in your Invoicing settings or upgrade your Odoo."))
             pdf_bin, unused_filetype = report.with_context(snailmail_layout=not self.cover, lang='en_US').render_qweb_pdf(self.res_id)