raise exception if token request(access) return error

I create yandex money app, which needed [client_secret](http://api.yandex.ru/money/doc/dg/reference/obtain-access-token.xml),
so when i obtained access token i started:
```shell
api.obtain_token
```
and had nil, without exception. (response["error"] = "unauthorized_client")
Notice : Of'course i know, that error can be unauthorized_client,invalid_grant, invalid_request , but for more consize raising, i raised only ``` YandexMoney::ApiError ```(no YandexMoney::UnauthorizedError and etc )