Add sched_error() through sched_debug3() functions.

These calls will replace all these:
	error("sched: message...")
with:
	sched_error("message...")

This allows the _log_msg function to stop calling xstrncmp() on every
log message while holding log_lock when SchedLogLevel > 0.
This behavior has effectively removed all concurrency from slurmctld when
SchedLogLevel is enabled, and prevented larger scale systems from being able
to start up - see bug 3746 for an example of this.