Add error messages to JSONReader and friends. This required a bit of refactor...

base/values.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file specifies a recursive data storage class called Value
 // intended for storing setting and other persistable data.
 // It includes the ability to specify (recursive) lists and dictionaries, so
 // it's fairly expressive.  However, the API is optimized for the common case,
 // namely storing a hierarchical tree of simple values.  Given a
 // DictionaryValue root, you can easily do things like:
@@ skipping 334 matching lines @@
 class ValueSerializer {
  public:
   virtual ~ValueSerializer() {}
 
   virtual bool Serialize(const Value& root) = 0;
 
   // This method deserializes the subclass-specific format into a Value object.
   // The method should return true if and only if the root parameter is set
   // to a complete Value representation of the serialized form.  If the
   // return value is true, the caller takes ownership of the objects pointed
-  // to by root.  If the return value is false, root should be unchanged.
-  virtual bool Deserialize(Value** root) = 0;
+  // to by root.  If the return value is false, root should be unchanged and if
+  // error_message is non-null, it should be filled with a message describing
+  // the error.
+  virtual bool Deserialize(Value** root, std::string* error_message) = 0;
 };
 
 #endif  // BASE_VALUES_H_