If you `ctrl + c` the Puppeteer parent process, we would sometimes
not properly handle killing of the child processes. This would then
leave child processes behind, with running Chromium instances. This
in turn could block Puppeteer from launching again and results in
cryptic errors.

Instead of using the generic `process.kill` with the process id
(which for some reason is negative the pid, which I don't get),
we can kill the child process directly by calling `proc.kill`.

On Windows, it might not always cleanup the process, as it was
already cleaned up. We can relax the check here.