This avoids using JavaScript exceptions to support `EventLoop::run_app`
on the web, which is a huge hack, and doesn't work with the Exception
Handling Proposal for WebAssembly:
https://github.com/WebAssembly/exception-handling
This needs the application handler passed to `run_app` to be `'static`,
but that works better on iOS too anyhow (since you can't accidentally
forget to pass in state that then wouldn't be dropped when terminating).
