[FIX] snailmail: change error code when timeout

When the snailmail API-call timed out, the SnailmailLetter.state and SnailmailLetter.error_code were not changed, which resulted in an infinite loop of retries  via the "Snailmail: process letters queue" cron job.
This commit changes this behavior: On a timeout the SnailmailLetter.error_code is changed such that no retry happens. Following stable policy, no timeout error is added, but 'unknown error' will be used. Preventing retries on timeout is mandatory as timed-out request are indeed processed by IAP and customer credited.

closes odoo/odoo#81659

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