Handle AnsibleUndefinedVariable's raised from lookup plugins like jinja2 UndefinedError's

This allows lookup plugins that fail to find a requested variable to fail the same way as simple jinja2 interpolation variable-not-found errors. Specifically, the error should be ignored when `fail_on_undefined` is false. In my case, my custom lookup plugin was failing when used as the `name` property of a task, which caused the whole playbook execution to incorrectly fail.