Quit with an error if cluster_settings is not valid

Same fix as https://github.com/Metaswitch/sprout/pull/810. No testing yet, but I'll:

* make sure Ralf can start up normally
* blank out cluster_settings, restart Ralf and check for an error log
* blank out cluster_settings, SIGHUP Ralf and check for an error log
* delete cluster_settings, restart Ralf and check for an error log