expose exchange status on record-an-exchange endpoint

I need this for the MassPay script because PayPal payouts can fail and we want to record that when we post back.
This block is wrong, charges only update the balance once they've succeeded, but payouts also update it when they're still pending. The right way to do it probably is to use the `record_exchange()` and `record_exchange_result()` functions in `gratipay.billing.exchanges`.