Wrap fetchList.parser usages in try catch

[JonasWanke]

Is your feature request related to a problem? Please describe. The assignments screen of the demo teacher reports a loading error which comes from DateTime.parse. If such errors occur only in a single entity, we should just report that single item as broken and still show other valid assignments.

Describe the solution you'd like Wrap the following call in try catch and somehow show that error to the user: https://github.com/schul-cloud/schulcloud-flutter/blob/862feb3bac23cb3a629cf124055cf7d28cfaeb46/lib/app/utils.dart#L136