Return a human-friendly error message when upload fails
    
    Instead of giving the object which won't tell anything to the user, give
    a human-friendly error message. It is not very descriptive but we do not
    have more info. So stick with this.
    
    Fixes https://pagure.io/pagure/issue/2367
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>