Don't promisify fs.exists

Don't promisify fs.exists since this does not play by the rules of node style callback params, where the first one should be err and then data. It seems this does return only the data, which promisify understands as an error and does throw right away. Instead do what the node js docs suggest too and use fs.access instead, like so: promisify(fs.access)(aPath, fs.R_OK). See it in action [here in the cosmowiki].