fix false notification/rebuild for dx hot-reload

[felixf4xu]

try to fix #1566

this PR is more like a workaround, and it might not be perfect. Any comment is welcome.

[ealmloff]

Thank you! I added one extra check to make sure the file exists. It appeared to work as is, but it isn't clear from notify's documentation if removing a file will trigger a changed path, so I would rather be safe than run into a panic later

[felixf4xu]

if let Ok is better