core: downgrade legit error logs (#5705)

core: downgrade legit error logs (#5705)

manager_sync_bus_names() function retrieves the dbus names
and compares it with unit bus names. It could be right
after the list is retrieved, the dbus peer is disconnected.
In this case it is really not an ERROR print if
sd_bus_get_name_creds() or sd_bus_creds_get_unique_name()
fail.