Emit a failing error code for failing validation.

test/lish/package_validation_has_an_error_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:pub/src/exit_codes.dart' as exit_codes;
 import 'package:scheduled_test/scheduled_server.dart';
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
   setUp(d.validPackage.create);
 
   integration('package validation has an error', () {
     var pkg = packageMap("test_pkg", "1.0.0");
     pkg.remove("homepage");
     d.dir(appPath, [d.pubspec(pkg)]).create();
 
     var server = new ScheduledServer();
     var pub = startPublish(server);
 
-    pub.shouldExit(exit_codes.SUCCESS);
+    pub.shouldExit(exit_codes.DATA);
     pub.stderr.expect(consumeThrough("Sorry, your package is missing a "
         "requirement and can't be published yet."));
   });
 }