In case there is some serious issue with some DNS zones, you can manually whitelist them. This should not be necessary in most cases and hence there is no way to do with on the web user interface.
Generally it is strongly advised against doing this or disabling DNSSEC altogether.
Domains can be whitelisted in /etc/sysconfig/unbound as shown here:
INSECURE_ZONES="abc.com xyz.com"
After modifying this file, unbound needs to be restarted.