ReadOnly builders must have a validation method

[octonato]

As explained by @ignasi35 here: #2 (review)

[ignasi35]

Please, review my further comments on that review. I don't think validation should be a separate concern.

[octonato]

Yes, that was my point for Command Handlers that do emit events.

That is not valid for a ReadOnly. We may need something that block the execution of the command, hence the validate method.

We can't do it in the replyWith method because we only have access to the State, not the Command. Otherwise we could throw an exception in the replyWith.