feat: support errors.Is(), errors.As() in RedisError

[Syuparn]

By this PR, internal errors wrapped by redsync.RedisError can be checked by errors.Is() and errors.As(). This helps errors handlings to check the error cause like below.

if err := mutex.Lock(); err != nil {
	if errors.Is(err, syscall.ECONNREFUSED) { // connection refused error wraps this
		fmt.Printf("connection refused: %v\n", err)
	}
	fmt.Printf("unexpected: %v\n", err)
	return
}

[hjr265]

Thank you for this PR, @Syuparn!