Add support for relative paths in the template lookup plugin for roles

Know the template lookup can be use in role. (add search in the role/templates/ and the playbook_dir)
I have add an error message if the template doesn't exist ( before the error was a empty string).

Same as https://github.com/ansible/ansible/commit/ad97c618cf82a1db2e242b8af039b4612294adde#diff-b4b03eaeefc82fb143509b97102a08ce but for the template lookup.