ApiErrorHandler.kt

class ApiErrorHandler(val context: Context?) {
    fun handle(throwable: Throwable) {
        // レスポンスコードとエラーレスポンスの中身をパースしてユーザにエラー内容を伝えるためのなにかをする
        if (throwable is HttpException) {
            ...
        }
    }
}

ExamplePresenter.kt

class ExamplePresenter(val view: ExampleView, val repository: UserRepository) {
    fun fetchName() {
        repository
            .fetchName()
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe({ result ->
                result
                   .onSuccess {
                        view.showName(it)
                   }
                   .onFailure {
                        view.showError(it)
                   }
            })
        ...
    }
}

interface ExampleView {
    fun showName(name: String)
    
    fun showError(throwable: Throwable)
}

ExampleResult.kt

sealed class ExampleResult<T> {
    data class Success<T>(val data: T) : ExampleResult<T>()
    data class Failure<T>(val throwable: Throwable) : ExampleResult<T>()
    
    inline fun <T> ExampleResult<T>.onFailure(action: (exception: Throwable) -> Unit): ExampleResult<T> {
    if (this is ExampleResult.Failure) {
        action(it)
    }
    return this
    }

    inline fun <T> ExampleResult<T>.onSuccess(action: (value: T) -> Unit): ExampleResult<T> {
    if (this is ExampleResult.Success) {
        action(this.data)
    }
    return this
    }
}

UserRepository.kt

import io.reactivex.Single

class UserRepository {
    // Single<Result<表示に必要な型>> を返す
    fun fetchName() : Single<ExampleResult<String>> {
        ApiClient().create(UsersService::class.java)
            .fetchName()
            .map {
                val body = it.body()
                if (it.isSuccessful && body != null) {
                    ExampleResult.success(body.name)
                } else {
                    ExampleResult.failure(HttpException(it))
                }
            }
    }
}