workqueue: handle nice better

workqueue: handle nice better

nice returns the program's schedule priority. This can be a negative
value when there is no error condition. To check if nice is triggering
an error we have to check errno.

The most recent three macOS test failures appear to be due to this
problem.

https://github.com/axboe/fio/actions/runs/6347762639/job/17243247222
https://github.com/axboe/fio/actions/runs/6346019205/job/17239370410
https://github.com/axboe/fio/actions/runs/6241934089/job/16944981876

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>