The exception handling is different depending on which ModuleManagerUI is used (in ModuleManagerNullUI, everything happens in onStartup(), which is called in ConfigLoader, while in ModuleManagerWebUI, everything happens in calls to execute(), which are called by the InstallFilter)

In both cases, Magnolia should not start if the install or update was unsuccessful. We only need to figure out what to with incoming http requests, when using ModuleManagerNullUI. This is very much related to MAGNOLIA-2244 which asks for a review/cleanup/removal of the ConfigLoader.isBootstrapping(), .isConfigured() and .enterListeningMode() methods !