// Model
fun getSomeData(page: Int, consumer: Consumer<Throwable>): Observable<List<SomeData>> {
    return getSomeDataFromApi(page)
            .doOnError(consumer)
            .onErrorResumeNext(getSomeDataFromDb(page))
}

// Presenter
fun loadSomeData(page: Int) {
    model.getSomeData(page, Consumer { view.showError(it.message ?: "Error!") })
            .subscribe { view.showSomeData(it) }
}