Refine extension-prefs' error UI
!1 (merged) includes some very crude error reporting, but I couldn't complain about it as it's if anything consistent with the existing awful UI.
So let's not be sloppy just because we hope that users don't get to see this:
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if no extensions are installed, but the user launches the prefs tool for some reason, show a proper blank state
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if a preference widget fails to load, hide the technical details behind an expander instead presenting the user with a window full of gibberish
There's also a flatpak bundle for convenient testing.