Make pub publish more user friendly:

utils/pub/validator/lib.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 lib_validator;
 
 import 'dart:io';
 
 import '../entrypoint.dart';
 import '../io.dart';
 import '../system_cache.dart';
 import '../utils.dart';
 import '../validator.dart';
 
 // TODO(nweiz): When issue 7196 is fixed, complain about non-Dart files in lib.
 /// A validator that checks that libraries in "lib/" (and not "lib/src/") exist
 /// and are well-formed.
 class LibValidator extends Validator {
   LibValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   Future validate() {
     var libDir = join(entrypoint.root.dir, "lib");
 
     return dirExists(libDir).chain((libDirExists) {
       if (!libDirExists) {
-        errors.add('Your package must have a "lib/" directory so users have '
-            'something to import.');
+        errors.add('You must have a "lib" directory.\n'
+            "Without that, users cannot import any code from your package.");
         return new Future.immediate(null);
       }
 
       // TODO(rnystrom): listDir() returns real file paths after symlinks are
       // resolved. This means if libDir contains a symlink, the resulting paths
       // won't appear to be within it, which confuses relativeTo(). Work around
       // that here by making sure we have the real path to libDir. Remove this
       // when #7346 is fixed.
       libDir = new File(libDir).fullPathSync();
 
       return listDir(libDir).transform((files) {
         files = files.map((file) => relativeTo(file, libDir));
         if (files.isEmpty) {
-          errors.add('The "lib/" directory may not be empty so users have '
-              'something to import');
+          errors.add('You must have a non-empty "lib" directory.\n'
+              "Without that, users cannot import any code from your package.");
         } else if (files.length == 1 && files.first == "src") {
-          errors.add('The "lib/" directory must contain something other than '
-              '"src/" so users have something to import');
+          errors.add('The "lib" directory must contain something other than '
+              '"src".\n'
+              "Otherwise, users cannot import any code from your package.");
         }
       });
     });
   }
 }