Show API errors in frontend

[devmount]

Is your feature request related to a problem? Please describe.
If the backend API throws an error (like 401, 403 or similar) those are currently not shown in the frontend (with a few exceptions I think).

Describe the solution you'd like
Use a frontend notification component to show error messages from backend to the current user, if it makes sense for the use case (e.g. the reason why appointment creation failed).

Additional context
https://github.com/thunderbird/appointment/pull/170/files?diff=unified&w=1#r1393343082

[MelissaAutumn]

@solangevalverde I can't find any designs that allow for a prettied up dump of red text on various forms. Can you take a look at this when you get a chance? This can technically occur anywhere we have a save or update button, but most notably during "Create new appointment", "Edit calendar" and "Account settings" flows.

Examples of errors can be found under the "Exceptions" section of the ftl file: https://github.com/thunderbird/appointment/blob/main/backend/src/appointment/l10n/en/main.ftl#L11

[MelissaAutumn]

I'm going to do an initial ugly pass on this