feat: make not-found errors discoverable

[Stebalien]

Now we just need a common place to put:

func IsNotFound(e error) bool {
  for e != nil {
    if ne, ok := e.(interface { NotFound() bool }); ok {
      return ne.NotFound()
    }
    if ue, ok := e.(interface { Unwrap() error }); ok {
      e = ue.Unwrap()
    } else {
      return false
    }
  }
  return false
}