Add failure messages to gin and js mojom encoder

gin/function_template.h

 // Copyright 2014 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 GIN_FUNCTION_TEMPLATE_H_
 #define GIN_FUNCTION_TEMPLATE_H_
 
 #include "base/callback.h"
 #include "base/logging.h"
 #include "gin/arguments.h"
@@ skipping 113 matching lines @@
 // at position |index|.
 template <size_t index, typename ArgType>
 struct ArgumentHolder {
   using ArgLocalType = typename CallbackParamTraits<ArgType>::LocalType;
 
   ArgLocalType value;
   bool ok;
 
   ArgumentHolder(Arguments* args, int create_flags)
       : ok(GetNextArgument(args, create_flags, index == 0, &value)) {
-    if (!ok)
+    if (!ok) {
+      // Ideally we would include the expected c++ type in the error
+      // message which we can access via typeid(ArgType).name()
+      // however we compile with no-rtti, which disables typeid.
       args->ThrowError();
+    }
   }
 };
 
 // Class template for converting arguments from JavaScript to C++ and running
 // the callback with them.
 template <typename IndicesType, typename... ArgTypes>
 class Invoker {};
 
 template <size_t... indices, typename... ArgTypes>
 class Invoker<IndicesHolder<indices...>, ArgTypes...>
@@ skipping 94 matching lines @@
   typedef internal::CallbackHolder<Sig> HolderT;
   HolderT* holder = new HolderT(isolate, callback, callback_flags);
   tmpl->SetCallAsFunctionHandler(&internal::Dispatcher<Sig>::DispatchToCallback,
                                  ConvertToV8<v8::Handle<v8::External> >(
                                      isolate, holder->GetHandle(isolate)));
 }
 
 }  // namespace gin
 
 #endif  // GIN_FUNCTION_TEMPLATE_H_