feat(auth): count PayPal invoice and SNS publish failures in StatsD
Because: - PayPal invoice failures and failed SNS publishes are only logged, so Yardstick has nothing to alert on This commit: - Counts paypal-processor.invoice.processed and .error per invoice - Counts notifier.publish.failure in the legacy notifier and NotifierService - Adds the matching unit test assertions Fixes PAY-3881