[FIX] mail: handle server error in the front-end part

Issue

    - Install Time Off
    - Marc Demo > Create leave request
    - Admin > Validate the leave request
    - Marc Demo > Add an attachment

    Nothing happens

Cause

    There is a access right issue that is not handled in the
    JS.

Solution

    Handle it by doing a do_warn.

OPW-2171791

closes odoo/odoo#43484

X-original-commit: 6a98c75e
Signed-off-by: Jason Van Malder <jvm-odoo@users.noreply.github.com>