Do not throw on missing directory/file.

[wtgtybhertgeghgtwtg]

From this comment.
Trying to watch a file that does not exist with node_watcher terminates the process. chokidar handles this by wrapping fs.watch in a try-catch block and swallowing the error if it's an ENOENT or ENOTDIR.
Does something like that sound okay?

[amasad]

Expanding on what I mentioned on #88:

1. internally watchdir is (and should) never be called with a non-directory.
2. If sane is called with a non-directory for a root that's an obvious user error that should be reported
3. Swallowing user errors is bad UX. Imagine if I had a typo, for example, it can be really hard to debug.