Add error/exception notifications for the user

[Robbendebiene]

Currently at lot of errors and exceptions are just swallowed in the code. (those places are marked with a TODO)
The goal of this issue is to add meaningful error messages (e.g. as toasts) that inform the user about what went wrong.

Resources:

• https://docs.flutter.dev/testing/errors
• https://api.flutter.dev/flutter/foundation/FlutterError/onError.html

[7h30n3]

Some messages were added in a0b767a

[Robbendebiene]

Missing error messages are:

• OSMAuthenticatedUserProvider on login/logout
• StopAreasProvider when loading stop areas

We could write a global error handler class/function that either contains all error messages with their respective errors or just handles and catches all uncaught errors (Unbekannter Fehler).

Option one would allow us to have less duplicated code for errors that can occur in multiple places like timeouts.
Also we do not have to write a try catch for every single place. Instead we handle them globally using one method
The disadvantage is that it becomes more obscure how the code will perform on an error.
So maybe we should start with option two (handle all uncaught errors)

Example using Flutters error handler: https://stackoverflow.com/questions/67762290/how-to-catch-async-exception-in-one-place-like-main-and-show-it-in-alertdialog
This also eliminates the problem of correctly retrieving the scaffold state/context.