When sending a message (e.g. in a CRM lead or SO),
a failure_type and failure_reason are displayed, even when the message
is successfully sent.
This is because these fields are not updated when the email_status
is changed to "sent". We reset these fields to '' when the message is sent.
OPW-2003290
closesodoo/odoo#34082
Signed-off-by: Nans Lefebvre (len) <len@odoo.com>