naive refactor of non-error checking in go

go1-err-example4.go

// GatherItems collects incoming items. If somehow we get more than n, an error is returned.
func GatherItems(n int) ([]string, error) {
	checkLen := func() {
		if len(items) > n {
			return items, fmt.Errorf("more than %d items gathered: %d", n, len(items))
		}
	}
	
	var items []string
	
	items = append(items, GetItemsFromSourceAlpha()...)
	checkLen()

	items = append(items, GetItemsFromSourceBeta()...)
	checkLen()
	
	items = append(items, GetItemsFromSourceGamma()...)
	checkLen()
	
	return items, nil
}