models/login_source: code improvement

models/error.go

@@ -635,6 +635,19 @@ func (err ErrLoginSourceAlreadyExist) Error() string {
 	return fmt.Sprintf("login source already exists [name: %s]", err.Name)
 }
 
+type ErrLoginSourceInUse struct {
+	ID int64
+}
+
+func IsErrLoginSourceInUse(err error) bool {
+	_, ok := err.(ErrLoginSourceInUse)
+	return ok
+}
+
+func (err ErrLoginSourceInUse) Error() string {
+	return fmt.Sprintf("login source is still used by some users [id: %d]", err.ID)
+}
+
 // ___________
 // \__    ___/___ _____    _____
 //   |    |_/ __ \\__  \  /     \