debugger: improve ESRCH error message

When using `iojs debug -p <pid>` with an invalid pid, the debugger
printed an internal error message because it wasn't smart enough
to figure out that the target process didn't exist.  Now it is.

PR-URL: https://github.com/nodejs/io.js/pull/1863


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>