Unloader hanging indefinitely due to a worker thread exception

[jcross-linkup]

We ran into an issue where the unloader would throw an exception, but instead of exiting non-0, it would hang indefinitely. Basically not all the threads were being shutdown properly, so the unloader got into a bad state with at least one thread waiting forever for more tasks. Forcing a system.exit fixed the issue.

We submitted a PR to fix the issue: #91