Catch and log spawn failures

> I have no idea why this `EPIPE` error is getting raised, or how to fix it. I'm spawning a child process using spawn `child.js` …

It seems quite a few people have run into this issue over the past three years, including myself. (#1)

First, the child process isn't getting spawned at all.  `spawn('child.js')` fails, but the `ENOENT` is masked, as the `EPIPE` is raised first.

`spawn('node', ['child.js'])` would work as intended, but why is `EPIPE` raised at all?

The lamented W. Richard Stevens writes in “Advanced Programming in the UNIX Environment”:

> If we `write` to a pipe whose read end has been closed, the signal `SIGPIPE` is generated. If we either ignore the signal or catch it and return from the signal handler, `write` returns −1 with `errno` set to `EPIPE`.

The attached patch shows how to catch both the `EPIPE` and the `ENOENT`.