Fix error reporting on invalid command line args.

utils/pub/command_help.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library command_help;
 
+import 'dart:io' as io;
+import 'exit_codes.dart' as exit_codes;
+import 'io.dart';
 import 'pub.dart';
 
 /** Handles the `help` pub command. */
 class HelpCommand extends PubCommand {
   String get description => "display help information for Pub";
   String get usage => 'pub help [command]';
   bool get requiresEntrypoint => false;
 
   Future onRun() {
     if (commandOptions.rest.isEmpty) {
       printUsage();
     } else {
-      pubCommands[commandOptions.rest[0]].printUsage();
+      var name = commandOptions.rest[0];
+      var command = pubCommands[name];
+      if (command == null) {
+        printError('Could not find a command named "$name".');
+        printError('Run "pub help" to see available commands.');
+        io.exit(exit_codes.USAGE);
+      }
+
+      command.printUsage();
     }
   }
 }