Perhaps I’m wrong, but I believe that upload occurs entirely on the Client, resulting in a Media Object in the FileLoader object’s file member (or files if you’re uploading multiple files) on the Client. Transmitting that to the Server is a separate step.
If the upload process fails, I believe it should raise a Python exception, in the Client. However, I have found no documentation saying so. Nor can I see which type of Exception that should be caught. (You may want to experiment with deliberately failing/cancelling an Upload to see what happens.)
In the meantime, I’ll add this omission (?) to the ongoing Documentation discussion.