Inform the user when changing the assignee failed
    
    We had an issue where the user asking to take or drop assignee on
    a ticket had no feedback if the ajax call failed.
    This commit fixes that by reporting an error to the user.
    
    Fixes https://pagure.io/pagure/issue/3996
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>