nice() error handling

I've attached a patch which fixes a potential issue I noticed while
reading the POSIX specs: nice() can succeed and return -1, so it's
recommended to set errno to 0 and check it afterwards:

"As -1 is a permissible return value in a successful situation, an
application wishing to check for error situations should set errno to 0,
then call nice(), and if it returns -1, check to see whether errno is
non-zero."

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