Add failure messages to gin and js mojom encoder

gin/function_template.h

Index: gin/function_template.h
diff --git a/gin/function_template.h b/gin/function_template.h
index 2b66b666ac8b76ed1dfa36686bab2d9eeac6650a..dff1bb2d7dce4f2e270efa62699eeb9ec2eee98b 100644
--- a/gin/function_template.h
+++ b/gin/function_template.h
@@ -131,8 +131,12 @@ struct ArgumentHolder {
 
   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();
+    }
   }
 };