Provide a nicer error message if something goes wrong when loading resources at startup

Currently when something goes wrong when starting up Ember, in most cases something to do with resource loading, nothing is shown and the application just exits. It would be nice to have a simple error message to show the end user. This would need to only depend on basic Ogre functionality, very much like the current loading bar. It should show a basic error message and proceed with the shutdown once the user has pressed a key.