Add optional schema compiler error messages

tools/json_schema_compiler/util.h

 // Copyright (c) 2012 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.
 
 #ifndef TOOLS_JSON_SCHEMA_COMPILER_UTIL_H__
 #define TOOLS_JSON_SCHEMA_COMPILER_UTIL_H__
 
 #include <string>
 #include <vector>
 
@@ skipping 155 matching lines @@
 }
 
 template <class T>
 scoped_ptr<base::Value> CreateValueFromOptionalArray(
     const scoped_ptr<std::vector<T> >& from) {
   if (from.get())
     return CreateValueFromArray(*from);
   return scoped_ptr<base::Value>();
 }
 
+std::string ValueTypeToString(Value::Type type);
+
 }  // namespace util
 }  // namespace json_schema_compiler
 
 #endif // TOOLS_JSON_SCHEMA_COMPILER_UTIL_H__