Handle errors consistently

[lgrahl]

At the moment, we sometimes use an alert and sometimes use the notification bar at the bottom. I'm not entirely sure we use both consistently. At least, we should define what to use for which error case.

[dbrgn]

With "alert" you mean a dialog box, and not an actual alert(), right?

The differentiation should probably be the same as on Android: Use a dialog when it's important that the user sees and confirms the issue, use the toast notification if it's a mere FYI.