`w4 run` crashes if the cart file is deleted
Running `w4 run` and then deleting the cartridge while it's running causes a crash.
For example, I ran `rm output/cart.wasm` during this run:
```
Matt@Matts-PC ~/p/w/w/wave-cycle (master)> w4 run --no-qr output/cart.wasm
Open http://localhost:4444. Press ctrl-C to exit.
✔ Reloaded cart.wasm
events.js:291
      throw er; // Unhandled 'error' event
      ^

Error: ENOENT: no such file or directory, open 'C:\cygwin64\home\Matt\prog\web\wasm4\wave-cycle\output\cart.wasm'
Emitted 'error' event on ReadStream instance at:
    at internal/fs/streams.js:136:12
    at FSReqCallback.oncomplete (fs.js:156:23) {
  errno: -4058,
  code: 'ENOENT',
  syscall: 'open',
  path: 'C:\\cygwin64\\home\\Matt\\prog\\web\\wasm4\\wave-cycle\\output\\cart.wasm'
}
Matt@Matts-PC ~/p/w/w/wave-cycle (master) [1]>
```
Which is a little annoying, as I like to delete all the stale build artefacts as part of my build process.
It would be nice if `w4 run` continued to watch the path and load the new file if one appears there.