Enhance the reaped status display

Instead of just printing a '_' for a job that has exited,
allow the user to tell if the job exited with an error or
not. A normally reaped job that ran to completion without
errors will have the '_' displayed. If the job exited with
an error, then an 'X' will be displayed instead.

Signed-off-by: Jens Axboe <axboe@kernel.dk>