let validateNumberFormat return if vatnumer is not EU

[ChielTimmermans]

Is it possible that validateNumberFormat will return err when it cannot find the landcode in patterns?
So change:

pattern, ok := patterns[n[0:2]]
if !ok {
	return false, nil
}

to

var CountryNotFound = errors.New("countryNotFound")

pattern, ok := patterns[n[0:2]]
if !ok {
	return false, CountryNotFound
}