This commit uses the `race` function from the async library to fix a bug
where exceptions raised from the server async wouldn't be caught by the
top-level code.

The bug was stemming from the fact that `runAllWorkers` is **blocking**,
despite its usage of `forConcurrently_`. Therefore we were never
actually running the `wait` function below, but rather we were hanging
waiting on the result of the first function.

As a result the server could die but the workers could keep the main
thread alive, causing the bug we just saw as part of #463.