I'm working on image support, specifically featured images/post thumbnails right now. I added file_is_valid_image() to my function. Then I had a thought.
If file_is_valid_image() returns false, CSV 2 POST creates a meta value (custom field) holding details about the problem.
Some people may think that it is the users responsibility to know if their data is valid or not. Problem is when something goes wrong, many users blame the plugin. They assume their data is fine, the plugin must be at fault. This idea is to protect CSV 2 POST.
It will also provide a service. I can create an interface to list such custom field values. The user can deal with them one at a time, deleting the meta as they go.
Could this be better than .txt log files which are outside of the WordPress core system?
I will add this system, starting with making the user aware when image URL are invalid. The ability to disable the feature and mass delete such meta values would will be added.