- Move 'handle_error_code_from_failed_htlc' to channel_db,
and call it from pay_to_route, because it should not be
called when HTLCs are forwarded.
- Replace 'payment_received' and 'payment_status'
callbacks with 'invoice_status' and 'request_status'.
- Show payment error logs in the Qt GUI
- In the invoices list, show paid invoices for which
we still have the log.