You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: datastore_test.go
    path of file: ./repos/gitkube/vendor/cloud.google.com/go/datastore
    
    the code of the file until where you have to start completion: // Copyright 2014 Google LLC
//
// Licensed under the Apache Lic
    
func checkErr(want string, err error) string {
	if err != nil {
		got := err.Error()
		if want == "" || strings.Index(got, want) == -1 {
			return got
		}
	} else if want != "" {
		return fmt.Sprintf("want error %q", want)
	}
	return ""
}