Crashing tasks don't kill Worker

[dktapps]

krakjoe/pthreads#724

Despite krakjoe's claim that this is expected behaviour, I think it's severely problematic:

• Memory issues on workers will crash all tasks that get submitted, without any indication to the main thread what's going on. This can happen due to, for example, overpopulation of thread-local storage.
• register_shutdown_function() is unable to capture fatal errors because it's never called (?!?!?!?!?!?)
• The parent thread has no idea that there might be a persistent state issue with a worker, causing it to continue to post tasks that subsequently fail too

One possible option is to just scrap Worker and reimplement it in userland (this isn't actually terribly difficult).

[dktapps]

Fixed by 5c595c1.