Add comments to templates used for testing (containing intentional errors)

[nicoavn]

The following template should include a comment indicating that it's not a proper usage, and rather a test that expects an exception coming from the call: https://github.com/level12/keg/blob/5ef0e17ed4829191a29ee37a6427a9b3754641dd/keg_apps/templating/templates/assets_with_params.html