bikeshedding: [single exception in an ExceptionGroup] to make it clear it is a single exception, and avoid someone thinking "ok and about the other exceptions in that exception group"?

One problem with appending the [single exc...] is it getting lost if the error message is very long and gets truncated, making it impossible to distinguish a wrapped exception from a non-wrapped exception.

FAILED foo.py::test_bar - ValueError: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa...

so unless we want custom logic for the truncation in _format_trimmed/_get_line_with_reprcrash_message it's probably better to prepend any message.

(you don't get truncation if you run in CI or with -vv, but day-to-day users will encounter it)

I think it's okay to truncate here; I'd rather lose the note than the actual message at least - and the group will still be visible if you go look at the specific test.