RESPONDER: Log failures to resolve user names in csv_string_to_uid_array
    
    This patch makes it more discoverable for the admin to find typos in the
    various user lists. Typically, the user lists are used to add access to
    some feature and printing a syslog message would make sure the admin
    sees the mistake.
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>