[FIX] point_of_sale: allow connect proxy https error

Since 79af654fc61 if we had unexpected configuration of usage such as
being in HTTPS, having a POS hardware in HTTPS but an error is received
(eg. the device is closed) => the POS interface can't be opened being
blocked on an error:

 Https connection to IoT Box failed
 Make sure you are using IoT Box v18.10 or higher.

 Navigate to {proxy_ip} to accept the certificate of your IoT Box.

With this changeset, we have a popup that does not prevent to open the
point of sale interface (and  the red disconnected status in the top
left allow to retry connection as before) on first load.

opw-1934413
closes #32306

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>