Add IsNotFound error check

Added function for checking if an error has code 404 (is not found).

httperror/error.go

@@ -105,6 +105,14 @@ func IsAPIError(err error) bool {
 	return ok
 }
 
+func IsNotFound(err error) bool {
+	if apiError, ok := err.(*APIError); ok {
+		return apiError.Code.Status == 404
+	}
+
+	return false
+}
+
 func IsConflict(err error) bool {
 	if apiError, ok := err.(*APIError); ok {
 		return apiError.Code.Status == 409