af22de update_release_notes: fix rpm --checksig error handling

Authored and Committed by Christian Boltz 3 years ago
    update_release_notes: fix rpm --checksig error handling
    
    The rpm output for a valid rpm signature changed since SLE 11.
    
    Instead of updating the grep pattern (and see it break again next year),
    switch the check to use $?
    
    Also log the full rpm output on error to make debugging easier.