Better 'missing/invalid settings.json' behaviour

When you run the ros bridge or the simulator, and the settings.json file is not found, somehow some defaults are found. This results in very difficult debugging. Like I just spent 30 minutes wondring why i got an error, then to find that my settings.json was missing.

Lets add a fatal error on missing settings.json with an error message that gives instructions on where to place the settings.json.