Coroutine exceptions: how do they propagate and when to use `CoroutineExceptionHandler`?

Answer

In structured concurrency, a failing child coroutine typically cancels its parent (unless you use `SupervisorJob`). In `launch`, an uncaught exception goes to the parent/handler; in `async`, the exception is kept until you `await()`. Use `try/catch` for local handling; use `CoroutineExceptionHandler` mainly for top-level `launch` to log/translate crashes.