readOrError.scala

import play.api.libs.json._
import play.api.libs.functional.syntax._
import play.api.data.validation.ValidationError

implicit class JsPathPimps(path: JsPath) extends JsPath {
  def readOrError[T](error: => String)(implicit r: Reads[T]): Reads[T] = new Reads[T] {
    def reads(json: JsValue): JsResult[T] = path.readNullable(r).reads(json) match {
      case JsSuccess(Some(value), _) => JsSuccess(value, path)
      case JsSuccess(None, _)        => JsError((path, ValidationError(error)))
      case err@JsError(_)            => err
    }
  }

  def formatOrError[T](error: => String)(implicit r: Format[T]): OFormat[T] =
    OFormat(readOrError(error), Writes.at(path)(r))
}

implicit val reader = (
  (__ \ 'name).readOrError[String]("Username undefined") and
  (__ \ 'email).readOrError[String]("Email undefined")
).tupled

val jsonTest = Json.obj("name" -> "julien")
jsonTest.validate(reader)

> JsError(List((,List(ValidationError(Email undefined,WrappedArray())))))