Wrap a common "cast error" for To___E functions

[bcongdon]

To make it easier to use Go1.13's errors.Is and errors.As, it'd be great if the To___E functions wrap a common "cast error" so that transitive callers to these functions can determine if an error comes from casting.

A slightly-silly example usecase:

func main() {
  val, err := maybeCast("not_an_int")
  if errors.Is(err, cast.Err) {
    fmt.Println("unable to cast!")
  }
  fmt.Println(val)
}

func maybeCast(myVal string) (int, error) {
  if someCondition() {
    return cast.ToIntE(myVal)
  }
  return fmt.Errorf("unrelated error")
}

If there's interest, I'd love to make a PR. :)