Without this, emrun web server will be left running after user code
quits the page abnormally without calling `exit()` (e.g. due to a page
awwsnap crash), and there are multiple processes in use by the browser.
emrun has a graceful fallback to weaker browser process detection when
psutil is not available, so this is easy to miss if not running emrun
with --verbose:
814ec05f74/emrun.py (L330-L339)