underlay action needs to handle a missing file

In the "Underlay: place a file image under the current image (background)" action, you can specify a file to use as the underlay. If this file does not exist, the final image is not created and you will get a broken image icon.

Ideal behaviour would be to display a drupal_set_message error and then just use plain white, I guess. That way at least the final image will still be created.