Failure.kt

import io.reactivex.Observable
import io.reactivex.functions.Consumer

class MyErrorHandler : Consumer<Throwable> {
    override fun accept(t: Throwable?) { TODO() }
}

class Foo {
    fun main() {
        val errorHandler = MyErrorHandler()
        Observable.just("foo").subscribe(this::stringConsumer, errorHandler)
    }

    fun stringConsumer(value: String) { TODO() }
}