Exit immediately if --parent-pid doesn't look like an integer.

If you pass "--parent-pid foobar", then we immediately log an error
message and exit. This check happens via `parseInt` rather than testing
for exceptions in `process.kill(parentPid, 0)` because `process.kill`
converts its pid argument to an integer, so `process.kill('foobar', 0)`
returns true just as `process.kill(0, 0)` does.