Failed order notification to customer and admin

**Is your feature request related to a problem? Please describe.**
If there's a failed order during checkout, there's no notification of this. Following up and assisting on failures could increase sales.

**Describe the solution you'd like**
Failed order notification to customer and admin. We have failed recurring payment notifications, but nothing for initial orders. WooCommerce has a good model to follow for this in their emails.

**Describe alternatives you've considered**
Having the gateway handle this, since some failing order notifications are not legitimate.