I anticipate that at one point https://github.com/iqb-berlin/testcenter-frontend/issues/53 will be solved, so in most serious error-casses, we would leave app and goto an error page. Since this functionality is missing, the error-handling at this point can only lead to clear messages, but let app continue to function (which does not make much sense at all).

closes https://github.com/iqb-berlin/testcenter-frontend/issues/165 for now.