Don't die if a watched file is deleted

[jtrussell]

Kinda, annoying.

[jtrussell]

Note that this only seems to happen when the containing folder of a watched file is killed.

E.g.

buttle -w foo/bar.md

# separate process
rm -rf foo

# oh no! buttle crashed!

[jtrussell]

And here's what I get for a stack trace:

events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: watch null EPERM
    at exports._errnoException (util.js:855:11)
    at FSEvent.FSWatcher._handle.onchange (fs.js:1296:21)

[jtrussell]

Seems relevant: nodejs/node-v0.x-archive#4337

[jtrussell]

Note sure there's a great way to handle this... hmmm...