[browser][MT] asynch dispatch soundness

make sure that functions passed to `emscripten_dispatch_to_thread_async`
- don't throw JS exceptions
- don't throw managed exceptions
- don't leak JS interop stack frame
- sync version propagates errors or error codes
- `invoke_later_when_on_ui_thread_sync` and `invoke_later_when_on_ui_thread_async` should catch all including "memory access out of bounds"