Should customize error message

[lucasdaiki]

When a entity is not valid, the error property is:

{ property1: { errors: [ 'Required undefined `property1` was not specified in `FakeEntity`.' ] } }

But the error message is not friendly to application user.

I suggest to add a new param in validation:

class FakeEntity extends Speck {
    static SCHEMA = {
        property1: {
            validator: Proptypes.number.isRequired,
            errorMessage: 'Property1 must be a number'
        }
    }
}

So, the error message will be:

{ property1: { errors: ['Property1 must be a number' ] } }