Catch exception while building due to instance being deleted

In run_instance() a instance_update() call in _start_building() can
result in a UnexceptedTaskStateError if the instance gets deleted
while being built. Since this is an expected situation, we don't
want this exception to be written in the logs. Instead catch this
and raise a proper BuildAbortException that gets handled above this
call.

Change-Id: Ib9d3dd22b33b104f1593d534876a5387df81d11c
Closes-Bug: #1223843