Provide user feedback when subscribing to invalid calendar

How to use GitHub

• Please use the 👍 reaction to show that you are affected by the same issue.
• Please don't comment if you have no relevant information to add. It's just extra noise for everyone subscribed to this issue.
• Subscribe to receive notifications on status change and new comments.

Steps to reproduce

1. Open the calendar app
2. Click New calendar (+) then New subscription from link (read only)
3. Give https://discuss.afpy.org/discourse-post-event/events.ics (but as it could change so I'm hosting a mirror of the current version here: https://mdk.fr/x/events.ics)

Expected behaviour

As the ICS contains an invalid line (line too long) I expected an error to be shown, maybe at subscription time, and then if an error occur later, I expect a red ⚠ near the calendar to tell the user this subscription cannot be fetched and why, so the user can fix or report the issue upstream.

Actual behaviour

Nothing happen, the calendar gets added with the other, and as no events is parsed no even are displayed. No errors are reported (neither in the logs or the user interface).

Server configuration

No idea, I'm not admin on the machine ☹

Client configuration

Browser: Firefox 96.0.1
Operating system: Debian bookworm

Logs

Logs tells it cannot be parsed, but don't tell why, we can learn more by removing the broad except here:

https://github.com/nextcloud/server/blob/215aef3cbdc1963be1bb6bca5218ee0a4b7f1665/apps/dav/lib/CalDAV/WebcalCaching/RefreshWebcalService.php#L274-L278

Start at apps/dav/lib/CalDAV/WebcalCaching/RefreshWebcalService.php around lines 274-278 and reproduce the issue with the supplied ICS URL. Trace how parsing failures reach the calendar UI; done means invalid subscriptions produce visible feedback explaining the failure both when added and when refreshed.