src: don't print garbage errors

If JS throws an object whose toString() method throws, then Node
attempts to print an empty message, but actually prints garbage.
This commit checks for this case, and prints a message instead.

Fixes: https://github.com/nodejs/node/issues/4079
PR-URL: https://github.com/nodejs/node/pull/4112


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>