Fix error handling while deploying AppStream
Fix error handling while deploying AppStream

Setting an error with second_error->message is going to work poorly
when second_error has never been set non-NULL.

Related to #1845, although not necessarily the full solution.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #1867
Approved by: alexlarsson

flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak