Notify on webhook failures

We use a webhook to create imager service user account. If the webhook fails, the user is not receiving his credentials and the Order stays in Processing status. That's OK but not ideal.

Woo webhook mechanism is quite primitive in that it just logs the status should the webhook fails. The only logic it has is that if there were 5 consecutive failures for a webhook, it disables it. No notification either in this case. The number of failures leading to disabling can be configured though.

We should plug into the delivery mechanism ('woocommerce_webhook_delivery`) and send a notification when there's a webhook failure.

Start by locating the WooCommerce `woocommerce_webhook_delivery` hook and reviewing how webhook failures are currently logged and disabled. Determine what notification should be sent when delivery fails, including the disabled-after-failures case. Done means failures trigger the agreed notification without changing the existing order behavior.

Written by the indexing model from the issue text.