Improve messaging inside the application

[paule96]

Currently, the web app can only show a very limited amount of information after a user does something. We should integrate like a notification center where we can push errors and success notifications too. For the most errors the only way to know that they exist is to take a look into the browsers console. It should be easy to implement that.

Important: many places in code that should throw an error in the UI are currently not handled and just have a Todo where they should be. So we should implement them.