What I do is store the errors in session on the server side.
Then on the client side, I use the on upload done registered event to call a small script on the server to pull messages from session for the uploaded file.
I return back the message to the client and display the message in a dialog box.
I bet it's the "getTotalBytes()" method, but I'm not sure. If so, I should incorporate this into the upload process to not waste server process time of uploading.
Give it a try. If that doesn't work, you may need to submit a support ticket to them to add new features to the kooluploader. I wonder how they will answer this forum thread.