fix(core): improve app:update feedback and error handling

[joshtrichards]

• Resolves: #

Summary

Improve occ app:update's user-facing messages and per-app error handling, without changing its existing option contract.

Changes

• Clarify command and option descriptions and improve messages for:
  • disabled app-store access;
  • an unavailable default app store;
  • missing apps or command targets;
  • available updates;
  • successful and failed updates;
  • apps that are already up to date.
• Preserve explicit app-ID precedence over --all, as well as the legacy behavior where --showonly or --showcurrent without an app ID inspect all apps.
• Catch exceptions escaping Installer::isUpdateAvailable() so one failed check no longer aborts an all-app run; remaining apps continue processing.
• Return a nonzero exit code when an update check or update operation fails.
• Avoid duplicate failure messages when an update throws an exception.

Known limitation: ordinary app-store fetch failures are still collapsed upstream into an empty result by AppFetcher and/or false by Installer. Consequently, this command still cannot distinguish those cases from a genuinely up-to-date app or another valid empty result, such as an administrator-configured app allowlist filtering out all apps. Improving that distinction is out of scope here and can be addressed later as part of an AppFetcher/result-model refactor.

Output examples

Scenario	Before	After
Already up to date	calendar is up-to-date or no updates could be found	App "calendar" is already up to date
Update available, --showcurrent	calendar new version available: 5.3.0 (current version: 5.2.1)	App "calendar": 5.2.1 → 5.3.0 available
Successful update	calendar new version available: 5.3.0 calendar updated	App "calendar": update available (5.3.0) App "calendar" updated successfully
Update failure	calendar new version available: 5.3.0 Error: Download failed calendar couldn't be updated	App "calendar": update available (5.3.0) App "calendar" could not be updated: Download failed
Failed check during all-app run	(uncaught exception; remaining apps not processed)	App "calendar" could not be checked for updates: App store unavailable App "contacts": update available (2.0.0) App "contacts" updated successfully

The command returns exit code 1 when an update check or update operation fails, even if other apps succeed.

Compatibility: existing options remain unchanged.

TODO

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI