Fix: let fs.stat throw if a file does not exist (fixes #1296)

This fixes #1296.

There was a failing test, which tested the ignoring-feature on a non-existing fixture file (the file extension was missing).

I’ve also noticed that the "file ignored"-warning [doesn’t specify the `severity` property](https://github.com/eslint/eslint/blob/master/lib/cli-engine.js#L239)
How is this change being tested?