Error.Unwrap unexpectedly returns nil when all attempts are not performed

[JamieEdge]

If an unrecoverable error is encountered before the final attempt, the unwrapped value of the error returned by Do is nil. This behaviour is demonstrated by the following program.

package main

import (
	"errors"
	"fmt"

	"github.com/avast/retry-go/v4"
)

func main() {
	err := retry.Do(
		func() error {
			return retry.Unrecoverable(errors.New("some error"))
		},
	)

	fmt.Println(err)
	fmt.Println(errors.Unwrap(err))
}

The following output is observed when using v4.3.1.

All attempts fail:
#1: some error
<nil>

The following output is expected instead, where the last encountered error is returned.

All attempts fail:
#1: some error
some error

This is caused by Error.Unwrap not checking for nil values.

retry-go/retry.go

Lines 245 to 247 in affbf8f

	func (e Error) Unwrap() error {
	return e[len(e)-1]
	}