Errors nobody handles

If no handler takes an error:

• During a flush (an effect threw), the engine keeps running the remaining effects, then throws once the queues are empty. One error is thrown as it is; several are thrown together as an AggregateError with an errors array. The caller of flushSync() or batch() sees the throw.
• During the flush that runs in a microtask (a plain signal write outside an event handler), there is no caller. The error is passed to the global reportError function, which browsers surface as a window error event.
• Outside a flush and outside disposal, the error is thrown at the call site.
• Errors from several cleanups during one disposal are thrown together after all of them ran.