cli: more detailed "Request is not in JSON format" error
    
    When there's a server-side traceback upon cli API call, the Frontend
    logic returns some html-based error page which may contain some useful
    error message.  At least when we are debugging.
    
    So newly, when cli is run with --debug option, try to parse the web page
    (try beautifulsoup lib, if available) and dump it on stderr.
    
    Merges: #1966