Replace use of handle_schedule_error() with set_vm_state_and_notify()

Conductor task build_instances() currently uses error handling code in
scheduler driver to set instance error state and issue notifications.
There is a similar method in scheduler utils that does the exact same,
which a prior change converted to correctly send instance objects to
notifications send_update().

This in additon has the added benefit of decoupling conductor from
scheduler driver internals.

Change-Id: Iad9ecf99734fb4c200ba046da42128299a1162f2
Related-Bug: #1396324
Merge "Replace use of handle_schedule_error() with set_vm_state_and_notify()"