feature(client.go): define error message

[masahiro331]

Hi maintainers.

Thank you for great library.
I'd like to handling error log across versions.

e.g.

type logger struct{}

func (logger) Error(msg string, values ...interface{}) {
	// I don't want this implementation
	// if msg == "request failed" {
	// }
	
	// I want to implement this.
	if msg == RequestFailedMsg {
		// doSomething()
	}
	log.Logger.Errorw(msg, values)
}

func main() {
	retryClient := retryablehttp.NewClient()
	retryClient.Logger = logger{}
}

I like this library, Thank you.

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