base_services_start: convert mcelog exception to hcn-init
    
    We've had this 'exception' for mcelog.service failing in here for
    years. Looking into it, it seems to now be fixed:
    https://bugzilla.redhat.com/show_bug.cgi?id=1526725
    and hasn't happened in our official instances for years (I guess
    because they're all Intel boxes). However, we have a similar case
    on ppc64le with hcn-init.service failing spuriously:
    https://bugzilla.redhat.com/show_bug.cgi?id=1894654
    so I'm just converting it into a workaround for that instead. We
    could wire this up to be more sophisticated, with some kind of
    array or hash of services that are allowed to fail and more
    complex checking code, but let's not bother unless/until it's
    necessary.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>