error.go
func (e *Error) Error() string {
	sev := "W"
	if e.Severe {
		sev = "E"
	}
	return fmt.Sprintf("[%s] %s: %s", sev, e.Name, e.Detail)
}
func (e *Error) Error() string {
	sev := "W"
	if e.Severe {
		sev = "E"
	}
	return fmt.Sprintf("[%s] %s: %s", sev, e.Name, e.Detail)
}
// Error formats the enmime.Error as a string.