Log inability to load viewer

[JAicewizard]

• Resolves: No registered issue
• Target version: main

Summary

JS pages often have irrelevant errors in the console, this adds the explicit error making it easy to sysadmins to know the reason why this app isn't working

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required

[elzody]

Thanks a lot for your contribution. We are having a conference in September and it would be great if you would consider coming to hang out: https://nextcloud.com/conference-2025/. We can sometimes provide travel support in case you need assistance.

Nonetheless, the changes look good, so we can go ahead and merge them.
@JAicewizard

[elzody]

/backport to stable31

[elzody]

/backport to stable30

[github-actions]

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