use Validated instead of Xor in CommandH

[svalaskevicius]

as suggested by @Fristi:

btw have you considered to return ValidatedNel[String, List[Evt]] instead of Aggregate.Error Xor List[Evt]?
at CommandH (https://github.com/svalaskevicius/eventflow/blob/master/src/main/scala/Cqrs/EventFlow.scala#L14)

[Fristi]

👍

[svalaskevicius]

fixed in #25