If accountsservice isn't available on the system bus, then we can't
ask it for the user's parental controls settings, and we also can't
ask it whether it even has the malcontent extension. Since this is
not a real security boundary, fail open.
This can be dropped if we depend on a version of libmalcontent that maps
these errors to MCT_APP_FILTER_ERROR_DISABLED.
Resolves: https://github.com/flatpak/flatpak/issues/3902
Bug-Debian: https://bugs.debian.org/972138
Signed-off-by: Simon McVittie <smcv@collabora.com>