return some error message in global error handler

it is really bad practice for us not to return anything in case of
error. In \BlueLaTex,we always return an error message to briefly
explain what happened
extract configuration loader to own bundle

All bundles potentially use the configuration loader, and to avoid
cyclic dependencies, the best way to do it is to have a bundle whose
single responsibility is to register the configuration loader.
Thus, it will probably be the first bundle that is started and then
everybody is happy and can use it.