child_process: add debug and error details

This commit adds debug() calls to spawn() and spawnSync(), and
attaches additional information to Error objects.

Fixes: https://github.com/iojs/io.js/issues/720
PR-URL: https://github.com/iojs/io.js/pull/721


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>