This isn't necessary. Because we just log the error, we never match off of it,
we don't need any structure beyond String (or now Debug, which still gives us
a way to print the error). This is for the ergonomics of not having to
constantly write `.map_err(|e| format!("{e:?}"))`.
TaskHandle::close meant run_now may panic if the task was closed. Now, tasks
are only closed when all handles are dropped, causing all handles to point to
running tasks (ensuring run_now won't panic).
