ErrorHandlingFlow.scala

import akka.NotUsed
import akka.stream.scaladsl.Flow

case object NotZero

// Emits either an Int when zero, or a domain object indicating
// that the Int is non zero. Use this approach instead of relying
// on exceptions. Exceptions are for exceptional conditions,
// largely unanticipated, and they will terminate a stream.
val flow: Flow[Int, Either[NotZero.type, Int], NotUsed] = 
  Flow[Int]
    .map(n => if (n == 0) Right(n) else Left(NotZero))