frontend: detect ClientDisconnected errors
    
    The ClientDisconnected is just HttpException sub-class, so we already
    handle that.  It is though good to special-case it and properly dump
    what is happening to logs.
    
    I realized that previous commit stopped logging the HTTP errors to logs
    entirely (we only have error code in the access_log).  It is probably
    better to say explicit, and still have a single-line error for each
    non-200 error we send to users.
    
    Relates: #1600