fix(index): skip non-existing files
fix(index): skip non-existing files

This can happen when files have invalid names or they are being deleted
while thumbsup is running.

Fixes #106

thumbsup/node-thumbsup,rprieto/thumbsup,thumbsup/thumbsup,thumbsup/node-thumbsup,thumbsup/node-thumbsup,thumbsup/thumbsup,rprieto/thumbsup