[FIX] payment_stripe: fix traceback when error has no event

Issue

    - Install eCommerce
    - Add Stripe
    - Pay with invalid cart

    Traceback

Cause

    When I improved stripe error, I didn't checked if
    an error could have no event

Solution

    Check if the error has an event

OPW-2186385

closes odoo/odoo#44414

X-original-commit: 772c0274
Signed-off-by: Jason Van Malder (jvm) <jvm@odoo.com>