Show info dialog to user if loading of conversations fail on startup

[mahibi]

If loading the conversations fails on startup, the user only sees a blank screen (because there is no offline support yet) but has no idea what's going wrong.

There should be a dialog in case something goes wrong.

Furthermore:

• log error (but don't flood logfile in future...)
• show option to re authenticate
• show option to remove user?
• show hint what else could be checked