Bugfix in argparsing

gte/__main__.py

             logger.debug('P%d : %s' % (pid, str(prog)))
         generation += 1
     exit(0)
-else:
+elif 'output_dir' in args:
     # Generate
     if args.output_dir is not None:
         from PIL import Image
 
     w.save(args.output)
     logger.info('Image saved in %s' % args.output)
+else:
+    parser.print_help()
+    exit(1)
 
 exit(0)