Retrofit2、RxJava2 でエラー時のレスポンスをいい感じに Data Class に詰める

ErrorMessage.kt

package com.tomoyashibata.myapplication

data class ErrorMessage(
  val message: String,
  val errors: List<Error>
)

data class Error(
  val resource: String,
  val field: String,
  val code: String
)

GitHubService.kt

package com.tomoyashibata.myapplication

import io.reactivex.Single
import retrofit2.http.GET

interface GitHubService {
  // クエリパラメータ `q` が必須だがエラーハンドリングの確認のためにわざと取り除いている
  @GET("/search/users")
  fun searchUsers(): Single<Users>
}

MainActivity.kt

// コア部分だけ抜粋
package com.tomoyashibata.myapplication

import com.squareup.moshi.Moshi
import io.reactivex.android.schedulers.AndroidSchedulers
import io.reactivex.schedulers.Schedulers
import retrofit2.Retrofit
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory
import retrofit2.converter.moshi.MoshiConverterFactory
import timber.log.Timber
import com.squareup.moshi.JsonAdapter
import retrofit2.HttpException

fun searchUsers(): Unit {
  service.searchUsers()
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe({
      Timber.i("success!!")
      Timber.i(it.toString())
    }, {
      Timber.i("error!!")
      
      if (it is HttpException) {
        val adapter = Moshi.Builder().build().adapter(ErrorMessage::class.java)
        val errorMessage = adapter.fromJson(it.response().errorBody()?.string())
        Timber.e(errorMessage.toString())
      }
    })
}