sched: Introduce task_status().

sched: Introduce task_status().

Before struct task can be made private to sched.c we must eliminate
code which directly accesses the fields of this structure. The last
offender is ->error: in many places we check this field to detect
whether some task is in an error condition.

This patch provides a public accessor function, task_status(),
for this purpose. All users of ->error are modified to call this
function instead.