app: Do not share the Error object in all completion tasks
app: Do not share the Error object in all completion tasks

Completion on my system was triggering an assertion error:
`ostree_repo_open: assertion 'error == NULL || *error == NULL' failed`

command:
`flatpak build-init build-dir bar.foo.Baz org.freedesktop.Sdk//1.6 org.freedesktop.Platform//1.6

Closes: #1491
Approved by: alexlarsson

flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak