Handle `SystemExit` error separately from `BaseException`
    
    Previously, running `fedora-review` with no arguments or just `--help` would result in `ERROR: Exception down the road...` due to Python ArgumentParser invoking `sys.exit()`, which raises `SystemExit`, which is a `BaseException`. This is not really an error in this case, though, so it's somewhat misleading to the user.
    
     * src/FedoraReview/review_helper.py: Handle `SystemExit` error separately from `BaseException`