Add redundancy by removing external dependencies like third party resolvers. Running your own local resolver is as simple as:

  apt-get install unbound
  echo "nameserver 127.0.0.1" > /etc/resolv.conf

If you want to take advantage of a shared cache by using a third party resolver, shove this at the bottom of your /etc/unbound/unbound.conf:

  forward-zone:
      name: "."
      forward-addr: ip.of.recursive.resolver
      forward-first: yes

The "forward-first" option means that it will attempt to contact ip.of.recursive.resolver to do the query first, and if it fails because it is down, then it will do the lookup it's self.