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.
> The right way to do it probably is to use the `record_exchange()` and `record_exchange_result()` functions in `gratipay.billing.exchanges`.

I considered that, but there's extra behavior under those methods that we don't want in this endpoint: I don't think we want to [modify `last_*_result`](https://github.com/gratipay/gratipay.com/blob/83005baae2380ba0a1d866fbfc0038fa6296110c/gratipay/billing/exchanges.py#L325-L331), and we definitely don't want to [disallow negative balances](https://github.com/gratipay/gratipay.com/blob/83005baae2380ba0a1d866fbfc0038fa6296110c/gratipay/billing/exchanges.py#L333-L334) in this context.