Add optional schema compiler error messages

tools/json_schema_compiler/h_generator.py

Index: tools/json_schema_compiler/h_generator.py
diff --git a/tools/json_schema_compiler/h_generator.py b/tools/json_schema_compiler/h_generator.py
index 7cf6836fbf00ae8d92d79d166c46c056ea7c0665..3eca5a7e4cf52cd22e3ded29fbca8b4e2cc06a65 100644
--- a/tools/json_schema_compiler/h_generator.py
+++ b/tools/json_schema_compiler/h_generator.py
@@ -26,6 +26,9 @@ class _Generator(object):
     self._cpp_namespace = cpp_namespace
     self._target_namespace = (
         self._type_helper.GetCppNamespaceName(self._namespace))
+    self._error_parameter = ''
+    if self._namespace.generate_error_messages:
+      self._error_parameter = ', std::string* error = NULL'
 
   def Generate(self):
     """Generates a Code object with the .h for a single namespace.
@@ -228,14 +231,15 @@ class _Generator(object):
           .Comment('Populates a %s object from a base::Value. Returns'
                    ' whether |out| was successfully populated.' % classname)
           .Append('static bool Populate(const base::Value& value, '
-                  '%(classname)s* out);')
+                  '%s* out%s);' % (classname, self._error_parameter))
         )
         if is_toplevel:
           (c.Append()
             .Comment('Creates a %s object from a base::Value, or NULL on '
                      'failure.' % classname)
-            .Append('static scoped_ptr<%(classname)s> '
-                        'FromValue(const base::Value& value);')
+            .Append('static scoped_ptr<%s> FromValue('
+                    'const base::Value& value%s);' %
+                        (classname, self._error_parameter))
           )
       if type_.origin.from_client:
         (c.Append()
@@ -279,7 +283,7 @@ class _Generator(object):
         (c.Comment('Populates a %s object from a base::Value. Returns'
                    ' whether |out| was successfully populated.' % classname)
           .Append('static bool Populate(const base::Value& value, '
-                  '%(classname)s* out);')
+                  '%s* out%s);' % (classname, self._error_parameter))
           .Append()
         )
       if type_.origin.from_client:
@@ -338,7 +342,8 @@ class _Generator(object):
 
     c = Code()
     (c.Sblock('struct Params {')
-      .Append('static scoped_ptr<Params> Create(const base::ListValue& args);')
+      .Append('static scoped_ptr<Params> Create('
+              'const base::ListValue& args%s);' % self._error_parameter)
       .Append('~Params();')
       .Append()
       .Cblock(self._GenerateTypes(p.type_ for p in function.params))