Handling deleted calendars and calendar events

[gikf]

Discuss your ideas or share your views:

After deleting calendar or calendar event in Google calendar, the events and chapters linked to those will be left in limbo.

In these cases calendar api should return 404 error (https://developers.google.com/calendar/api/guides/errors#404_not_found), so it should be easy to distinguish when that happens and remove calendar_id or calendar_event_id. Maybe also send email to the instance owner.

Calendar portion is partially related to #1896