Updater Stability on Network Errors

[a-gerhard]

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Is your feature request related to a problem? Please describe.
Currently, the CLI updater does not check the availability of the app store (apps.nextcloud.com) before starting the upgrade. The occ upgrade command also does not check this, and it does not seem to display an error message when trying to upgrading apps.
With apps.nextcloud.com having been down right now, I just upgraded my Nextcloud from 23 to 24.0.4, and I ended up with half my apps disabled due to being incompatible. There was no indication of any error whatsoever. And I now need to wait for the app store to be available again before my users can use the nextcloud as usual.

Describe the solution you'd like

• Add error messages to the occ upgrade command when it cannot reach the app respository. Also show an error in the Nextcloud Apps UI when the repository is unavailable. Update successful is also a pretty misleading statement in case of the repository being unreachable, this should be changed to something appropriate (e.g., "app upgrade routines successful, but not all apps could be downloaded properly").
• Check availability of external resources before starting the nextcloud upgrade (right before the fist change to the system has been made)
• The best would be to even download all required resources (like app upgrades) before making any changes to the server at all, so the process can be cancelled if the repository becomes unavailable during the process.
• If users are located behind a firewall or otherwise need to be able to work without the repository, there should be a configuration option or CLI arguments to continue the process in case of unavailability.

Describe alternatives you've considered
There aren't really alternatives - each task in the list above can stand on its own and will improve this. I could check this manually, but I think that I should not be required to know what to check for, and I should trust the updater to have adequate checks in place.

Additional context
Add any other context or screenshots about the feature request here.