Refactor error page

[mpsonntag]

Currently, whenever this.reportError is invoked, after a cascade of event emits, the user
is redirected to "App.vue" which invokes "ErrorPage.vue" which is not a properly registered route.
Probably due to this setup, the user will be stuck on this page, the other links are not responsive any more
and the user has to reload the page to properly browse again.

Either this behavior has to be fixed or the usage of this.reportError should be refactored alltogether.