I am used to log.Warnf("err %v", err), but since we didn't implement formatter interface, the result is just struct ... it's quite a pain to call err.Error everytime

• https://golang.org/pkg/fmt/#Formatter

type Formatter interface {
        Format(f State, c rune)
}

• does %w need some extra handling?
• print string [error] Implement fmt.Formatter #62 #63
• print stack
• pass format to inner error and multi error
• it seems non standard formatter is also possible?