workers: fix invalid exit code in parent upon uncaught exception

Now worker.on('exit') reports correct exit code (1) if worker has exited
with uncaught exception.

Fixes: https://github.com/nodejs/node/issues/21707

PR-URL: https://github.com/nodejs/node/pull/21713


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>