appNG fails to start when an Error is thrown in InitialiterService#loadSite

When an error is thrown in Step 2 "Build application context" while calling applicationContext.refresh(), the whole plaform startup crashes.
The cause only appears in the Tomcat-Logs, while in the appNG.log we just see that the instance is shut down.

So we should consider catching Throwable here and log the error.
Furthermore, the appNG instance will work if there are other sites starting successfully.