fix: Continue setup even if a file to delete is not found (#663)

* Continue setup even if a file to delete is not found

For various reasons, `.gitkeep` may be deleted already.

(In my case, even though I followed the [Getting Started](https://quartz.jzhao.xyz) instructions exactly, my first run resulted in an `fatal: 'upstream' does not appear to be a git repository`)

If we try to delete `.gitkeep` again and don't ignore `ENOENT`, then the whole setup fails.

* Use fs.existsSync