fs: pass err to callback if buffer is too big

In fs.readFile, if an encoding is specified and toString fails, do not
throw an error. Instead, pass the error to the callback.

Fixes: https://github.com/nodejs/node/issues/2767
PR-URL: https://github.com/nodejs/node/pull/3485


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>