Fail ./x.py on invalid command

Make the ./x.py script fail when run with an invalid command, like:

  ./x.py nonsense

This helps in case of chaining multiple runs, eg.:

  ./x.py biuld && ./x.py test

src/bootstrap/flags.rs

@@ -136,7 +136,7 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`");
             None => {
                 // No subcommand -- show the general usage and subcommand help
                 println!("{}\n", subcommand_help);
-                process::exit(0);
+                process::exit(1);
             }
         };