cylc-restart: fix getting stuck on loading broken state file

If you restart a suite with an empty state file it currently errors and gets stuck in a halfway house, unable to shutdown or proceed. This is due to sys.exit calls rather than exceptions being raised in the load_tasks method.

This replaces the `sys.exit('sometext')` calls in the `load_tasks` method of `cylc-restart` with `raise Exception('sometext')`.

@hjoliver - please review.