frontend: unify the UI and API error handling
    
    This way we can handle most of the exceptions on one place.
    
    This also removes significant amount of traceback log entries from
    frontend.log/error_log because we fix the issue from 314cab8a69a where
    we started logging tracebacks for every HTTP (non-200) error.
    
    Thus, this fixes #1600.  The traceback noticed there was just caused
    by connection failure between client and server (error 400).  The
    situation can be reproduced by 'kill -9'ing the client while SRC.RPM is
    being uploaded.
    
    Fixes: #1600