doc: add example for file existence with fs.stat

Add an example on how to test if a file exists with fs.stat.
Also add a link to the Common System Errors.

Fixes: https://github.com/nodejs/issues/6752
PR-URL: https://github.com/nodejs/node/pull/8585


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>