fs: improve error message descriptions

1. Change "Bad arguments" error messages to a more helpful message
"options should either be an object or a string".

2. Make braces consistent.

3. Return meaningful error message from fs_event_wrap's
FSEvent's Start function.

PR-URL: https://github.com/nodejs/io.js/pull/1870


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>