Less strict error logging on PhantomJS exit

Do not log error if PhantomJS is closed with SIGTERM.

---
There have been some subtle code changes after 1.2.1 which make PhantomJS sometimes exit with SIGTERM signal received instead of cleanly with code 0, there's no need to log any error in situation like this.
I would consider rewriting to the following:
```
} else if (signal !== "SIGTERM") {
    var diagnostics = (code ? ("code: " + code) : "") + (signal ? "signal: " + signal : "");
```