Error handling & notification in UI

[t-kramer]

@praftery @urwahah

I added a few more simple notifications as feedback to users when something goes wrong while using the app (e.g. missing load data, no equipment selected, etc.). Whenever you have time, please double-check if we covered most potential errors.

So far, we have three different notifications: WARNING (calculation goes ahead), ERROR (no calculation possible) and SUCCESS (calculation completed). I think these should cover most cases. The source code is in utils/error_handling.py.