sealed class Response
data class Success(val body : Data) : Response
data class Failure(val error : Throwable) : Response
...
if (res is Success) {
    updateUi(res.body)
}

Observable.Transformer

fun <T> Response.fold(failure: (Throwbable) -> T, success: (Data) -> T) = when(this) {
    is Success -> success(data)
    is Failure -> failure(error) 
}