Custom rejection handler that returns JSON

JsonRejectionHandler.scala

case class ErrorMessage(message: String, cause: String)
object ErrorMessage {
  import spray.json.DefaultJsonProtocol._
  implicit val errorFormat = jsonFormat2(ErrorMessage.apply)
}
import spray.httpx.SprayJsonSupport._
implicit val jsonRejectionHandler = RejectionHandler {
  case MalformedRequestContentRejection(msg, cause) :: Nil =>
    complete(StatusCodes.BadRequest, ErrorMessage("The request content was malformed", msg))

  // add more custom handling here

  // default case that wraps the Default error message into json
  case x if RejectionHandler.Default.isDefinedAt(x) =>
    ctx => RejectionHandler.Default(x) {
      ctx.withHttpResponseMapped {
        case resp@HttpResponse(_, HttpEntity.NonEmpty(ContentType(`text/plain`, _), msg), _, _) =>
          import spray.httpx.marshalling
          resp.withEntity(marshalling.marshalUnsafe(ErrorMessage(msg.asString, "")))
      }
    }
}

This is part of official docs now: http://doc.akka.io/docs/akka-http/current/scala/http/routing-dsl/rejections.html#customising-rejection-http-responses