Catch exception while building due to instance being deleted

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

Change-Id: Ib9d3dd22b33b104f1593d534876a5387df81d11c
Closes-Bug: #1223843
Merge "Catch exception while building due to instance being deleted"