Add error Feedback on client side

[j-toscani]

Description (five sentences max.) If something does not work as expected, I need to give the user helpful information on what to expect.

Possible implementation

• Error Page
• Information cia Toast

Acceptance Criteria

• If a Page cannot be rendered due to missing data, display a 404 Error page
• If a Page cannot be rendered due to permissions, display a 403/401 Page
• If client actions cannot be performed, e.g. sending a Form, display a Toast with helpful information

Out of Scope What not to worry/think about.

Document optional improvements Optional. Could lead to a better feature.