Dismiss error messages for stop areas outside the view port

[Robbendebiene]

Currently any time the loading of a stop area fails an error is displayed.
This is not very useful when the user quickly panned over the stop area and isn't looking at it any more.

Currently if the user pans back to the stop area (whether an error occurred or not) the app will automatically try to (re)load it. So either the previous error is resolved and the elements will load successfully or the user will eventually receive the error message.

Hints and Ideas:
The code responsible for displaying the error messages: https://github.com/OPENER-next/OpenStop/blob/master/lib/view_models/home_view_model.dart#L566-L598

Perhaps to solve this the Stream should return/throw the errors: https://github.com/OPENER-next/OpenStop/blob/master/lib/view_models/home_view_model.dart#L442 On the other hand the stream is supposed to return element updates and is only loosely coupled to stop area queries.