Exit with 1 when requesting an invalid subcommand

Quark prints the available command list when no command or an invalid
command was requested. This can be considered an error and we should
exit with a status code != 0, so that e.g. CI pipelines can promptly
fail.

Author: lv-develer

bin/quark

@@ -20,6 +20,7 @@ def main():
         print("Available commands:")
         for cmd, _ in sorted(commands):
             print(" %s" % cmd)
+        sys.exit(1)
 
 if __name__ == "__main__":
     main()