unrecoverableError cannot be unwrapped with errors.Is()

[everactivejack]

See example (https://play.golang.org/p/SFwoFoqUi_h):

package main

import (
	"fmt"
	retry "github.com/avast/retry-go/v3"
	"github.com/pkg/errors"
)

var ErrTest = errors.New("test")

func main() {
	err := retry.Unrecoverable(ErrTest)
	err2 := fmt.Errorf("wrapping: %w",ErrTest)
	
	fmt.Println(errors.Is(err,ErrTest))
	fmt.Println(errors.Is(err2,ErrTest))
}

Outputs:

false
true

I think this just means that the errUnrecoverable needs to implement Unwrap(), which should be a pretty simple change of adding (see https://play.golang.org/p/LpiCrGbpEOD)

func (u unrecoverableError) Unwrap() error {
	return u.error
}