Return a better error if you forget to add the subcommand

before it would just throw and exception. Now it complains that no command
was found and says the only valid one is {'push-to-quay'}

hack/graph-util.py

@@ -471,7 +471,7 @@ def get_token(args):
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description='Utilities for managing graph data.')
-    subparsers = parser.add_subparsers()
+    subparsers = parser.add_subparsers(dest='command')
 
     push_to_quay_parser = subparsers.add_parser(
         'push-to-quay',
@@ -489,10 +489,11 @@ def get_token(args):
         '--versions',
         help='Comma Seperated Versions to sync',
     )
-    push_to_quay_parser.set_defaults(action='push-to-quay')
 
     args = parser.parse_args()
 
-    token = get_token(args=args)
-    if args.action == 'push-to-quay':
+    if args.command == 'push-to-quay':
+        token = get_token(args=args)
         push(directory='.', token=token, push_versions=args.versions)
+    else:
+        parser.print_usage()