fs: do not pass Buffer when toString() fails

Even though an Error object is passed to the callback when readFile()
fails due to toString() failing, it is a bit strange to still see
data passed as the second argument. This commit changes that and only
passes the Error object in that case.

PR-URL: https://github.com/nodejs/node/pull/9670


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>