Validate that files exist

[nzakas]

Right now, if you pass in a file that doesn't exist to eslint, it explodes. We need to handle this nicely and exit with a non-zero exit code. Having a nice message like "Could not find file foo/bar.js" is a good idea.

Perhaps it also shouldn't continue if any one of the files can't be found, so make this a pre-pass before doing any actual linting.