Improve signal handling for error situations.
Improve signal handling for error situations.

If Bud#schedule_shutdown() has been called but EM is not running (e.g.,
because an exception occurred and EM crashed), shutdown immediately,
rather than trying to schedule an EM callback to do shutdown.

This improves Ctrl+C behavior: previously, we'd hang on EM failure
and ignore Ctrl+C from the user.

bloom-lang/bud