Related to Michael's suggestions: (https://github.com/ansible/ansible/issues/3658#issuecomment-21535516), I've updated my playbook.yml to match suggestions. Unfortunately, I'm still not seeing the expected results. I expect to see bippy fail. fake-inventory is just localhost

playbook.yml

---
- name: webserver plays
  hosts: all
  user: nathan
  sudo: no
  gather_facts: false

  vars:
    foo: "onetwothree"
    bar: "fourfivesix"
    err_variable_not_present: |
        Required variables not present. Make sure you have ~/.wa-dev-ansible-vars defined
        Use playbooks/group_vars/default as a starting point

  pre_tasks:
    - name: "ensure variables"
      fail: >
        msg="{{ err_variable_not_present }}"
      when: item is not defined
      with_items:
        - "{{ foo }}"
        - "{{ bar }}"
        - "{{ bippy }}"

results

$ ansible-playbook -vvv -i fake-inventory playbooks.yml 

PLAY [webserver plays] ******************************************************** 

TASK: [ensure variables] ****************************************************** 
skipping: [localhost] => (item=onetwothree)
skipping: [localhost] => (item=fourfivesix)
skipping: [localhost] => (item={{bippy}})

PLAY RECAP ******************************************************************** 
localhost                  : ok=1    changed=0    unreachable=0    failed=0