w4 run crashes if the cart file is deleted

[majaha]

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.