The app depends on showing the error message from the signal [via `NSLocalizedFailureReasonErrorKey`][er]. Could you build up the `NSError` in a [similar manner to the old code][oc]?

```objc
failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    [subscriber sendError:[NSError errorWithDomain:@"com.example"
                                           code:error.code
                                       userInfo:@{
                                                  NSLocalizedFailureReasonErrorKey: [operation.responseObject valueForKey:@"error"]
                                                  ?: @"Failed to create account"
                                                }]];
}];
```

[er]: https://github.com/andrask/RACMobiDevDay/blob/3b81d248d6f8bcdb0f16206622edcb5047c18124/RACMobiDevDay/SignUpViewController.m#L81-L84
[oc]: https://github.com/andrask/RACMobiDevDay/blob/ee4473830d6b2eda6a0adbc0af4effc21e374dc7/RACMobiDevDay/APIClient.m#L41-L52