handleRejections

val totallyMissingHandler = RejectionHandler.newBuilder()
  .handleNotFound { complete((StatusCodes.NotFound, "Oh man, what you are looking for is long gone.")) }
  .handle { case ValidationRejection(msg, _) => complete((StatusCodes.InternalServerError, msg)) }
  .result()
val route =
  pathPrefix("handled") {
    handleRejections(totallyMissingHandler) {
      path("existing")(complete("This path exists")) ~
        path("boom")(reject(new ValidationRejection("This didn't work.")))
    }
  }
 
// tests:
Get("/handled/existing") ~> route ~> check {
  responseAs[String] shouldEqual "This path exists"
}
Get("/missing") ~> Route.seal(route) /* applies default handler */ ~> check {
  status shouldEqual StatusCodes.NotFound
  responseAs[String] shouldEqual "The requested resource could not be found."
}
Get("/handled/missing") ~> route ~> check {
  status shouldEqual StatusCodes.NotFound
  responseAs[String] shouldEqual "Oh man, what you are looking for is long gone."
}
Get("/handled/boom") ~> route ~> check {
  status shouldEqual StatusCodes.InternalServerError
  responseAs[String] shouldEqual "This didn't work."
}