fixes for first_found lookup plugin

This patch lets first_found look up templates as well, earlier it would look just under rolename/files.
It also gives the user a clearer error - when the input file isn't found, compared to current error : 'input file not found at /ansible/roles/test-role/templates/None or /ansible/None'

I hope I've got the process for the PR right, not very confident as yet :|