Define a `NotHandle` type, that implements `std::error::Error`, and use
it as the error type in `HandleOrNull` and `HandleOrInvalid`.

Also, make the display messages more specific, and remove the `Copy`
implementation.

This test's expected stderr now includes a count of the number of types
that implment `Error`. This PR introduces two new types, so increment
the number by two.

This test has an expected stderr containing text like this:

```
help: the following other types implement trait `std::error::Error`:
             ...
           and 43 others
```

However, the number 43 is platform-specific; on Windows, there are two
additional types, `InvalidHandleError` and `NullHandleError`, and the
number of 45. So for now, disable this test on Windows.