errors.go
func IsOverQuota(err error) bool {
	callErr, ok := err.(*internal.CallError)
	return ok && callErr.Code == 4
}
func IsOverQuota(err error) bool {
	callErr, ok := err.(*internal.CallError)
	return ok && callErr.Code == 4
}
// IsOverQuota reports whether err represents an API call failure
// due to insufficient available quota.