Tidy check for io::ErrorKind::Other in std code

[Mark-Simulacrum]

We have a note in the ErrorKind documentation:

This ErrorKind is not used by the standard library.

but in practice, there are cases were we do make use of it, whether in tests or by accident. We should add a check in tidy that the ErrorKind::Other string doesn't show up in std code, with an ability to opt-out similar to style checks (probably on the same line only would be best).

The ideal of course is a compiler check but I don't think there's much we can do there without a custom lint, which seems like way too much effort. The tidy check can miss things (e.g., glob imports), but that's OK.