Errors when unsubscribing from a calendar while it's syncing

[hrb-hub]

Describe the bug
Unsubscribing from a calendar while it's being synced causes different errors to appear.

To Reproduce
Steps to reproduce the behavior:

1. Login to the same account on multiple clients (ideally different devices: Desktop, iOS, Android)
2. Subscribe to an external calendar with a lot of events.
3. Update a lot of events on the external calendar.
4. Start sync of the external calendar.
5. While clients are syncing, unsubscribe from the calendar.
6. See errors.

Expected behavior
Unsubscribing from an external calendar while it's syncing should be handled gracefully and not show errors.

Screenshots

Error message:
404: | GET /rest/tutanota/calendarevent/OHrZp1c--7-4/MTczOTUyNTcyNzM2MA
Stacktrace:
NotFoundError: 404: | GET /rest/tutanota/calendarevent/OHrZp1c--7-4/MTczOTUyNTcyNzM2MA
    at mo (https://assets.tutanota.com/common-min-Ct81WxnQ.js:1:37031)
    at suspensionHandler.isSuspended.p.onload (https://assets.tutanota.com/mail-worker.js:154:18573)
App version:
264.250129.0
Client type:
Android

Desktop (please complete the following information):

• OS: Linux
• Version 264.250129.0

Smartphone (please complete the following information):

• OS: Android
• Version 264.250129.0