dhcp6_get_duid() already handles failure to generate the DUID in a
sensible manner. No reason to duplicate the error handling in
generate_duid_from_machine_id().

Especially, because generate_duid_from_machine_id() used to cache the
random DUID in memory and reuse it from then on. There is no reason to do
that, /etc/machine-id must be available to NetworkManager. We still
handle such a grave error gracefully by generating a random DUID.