Backport !1209 “gs-fedora-third-party: Let the 'list' call fail only when the call itself failed” to gnome-41

Like with other functions, let it fail only if a call to the 'fedora-third-party' really failed. It's not a problem when no repo is configured.

Closes #1644 (closed)

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Trivial backport of !1209 (merged) to gnome-41.