[FIX] snailmail: better tests & delivery address now visible on report

Tests now check for the 'error' field in Pingen repsonse
snailmail_external_layout.js now targets the right div

closes odoo/odoo#76246

Task: 2588145 & 2583718
Signed-off-by: Florian Daloze (fda) <fda@odoo.com>