chromium/update: Fix fetching previous version.

After refactoring the updater we no longer did properly propagate the
exit code from the nix-prefetch-url call to the main script. So if the
newest version could not be fetched it didn't even bother to try the
previous release and we would end up with an empty hash.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
fixup! XXX: HYDRA PICKUP, DO NOT MERGE!