Correct pthread_setschedparam usage errors.

The documented name for the priority scheduling parameter is sched_priority (see, e.g. http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_setschedparam.html, http://www.kernel.org/doc/man-pages/online/pages/man3/pthread_setschedparam.3.html).

pthread_setschedparam reports the error code via the return value, not errno.