s3: Simplify IsNotExist()
s3: Simplify IsNotExist()

restic/restic,middelink/restic,ar-jan/restic,middelink/restic,restic/restic,kurin/restic,fawick/restic,jayme-github/restic,ar-jan/restic,jayme-github/restic,fawick/restic,kurin/restic