ifupdownmain: scheduler: propagate sched error on interfaces to upper layers

ifupdownmain: scheduler: propagate sched error on interfaces to upper layers

Ticket: CM-7168
Reviewed By: julien, nikhil, stannous
Testing Done: Tested with errors in interfaces file

This patch changes a few errors to warns. and propagates errors
on ifaceobjects to upperlayers.

- any exception passed to upper layers (/sbin/ifupdown) results in
exit code of 1

- It uses a global SCHED flag to flag a scheduler error (maybe there is
  a better way). But traversing all the interfaces again to check status
is an overkill

- Changes a few errors to warns

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>