I vaguely remember reading about conversation of requiring an input variable to exist , if it does not exist. Playbook should exit.
Please can someone point me to the documentation . (or maybe i was just dreaming about this feature existing) 
If you want to do this generically, look at examples/ansible.cfg in github for the configuration parameter to use.
Requires Ansible 1.3.
There is also a {{ foo | mandatory }} for specific variables.