foo.yaml

---
- name: test
  hosts: all
  gather_facts: false
  connection: local
  vars:
    foo: ~

  tasks:
    - name: foo
      debug:
        var: foo

    - assert:
        that: nerf is defined
        
        
Using /Users/jkeating/src/derp/ansible.cfg as config file

PLAYBOOK: foo.yaml *************************************************************
1 plays in foo.yaml

PLAY [test] ********************************************************************

TASK [foo] *********************************************************************
task path: /Users/jkeating/src/derp/foo.yaml:10
ok: [haderp] => {
    "foo": null
}

TASK [assert] ******************************************************************
task path: /Users/jkeating/src/derp/foo.yaml:14
fatal: [haderp]: FAILED! => {"assertion": "nerf is defined", "changed": false, "evaluated_to": false, "failed": true}

NO MORE HOSTS LEFT *************************************************************
	to retry, use: --limit @foo.retry

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

exit 2

ansible-playbook 2.1.0.0
  config file = /Users/jkeating/src/derp/ansible.cfg
  configured module search path = Default w/o overrides