Improve Bitstamp's exception handling

Use rescu's new exception handling functionality for Bitstamp.

Bitstamp returns http status 200 on exceptions, but rescu now supports parsing this as exception. Done for `BitstampAuthenticated`; could also be done for other interfaces by just adding `BitstampException` to the throws clause (assuming the error response is always in the form `{"error": "<error message>"}`.

Technically, this breaks `BitstampAuthenticated`'s public api, that's why I'm not pushing this directly into develop. (Eg. if a user is `catch`ing a `HttpStatusIOException` that was thrown previously, it won't work any more since `BitstampException` will be thrown now.)