IDL: Update type error message for invalid union values

Source/bindings/templates/union.cpp

Index: Source/bindings/templates/union.cpp
diff --git a/Source/bindings/templates/union.cpp b/Source/bindings/templates/union.cpp
index 3c9fbd493870146dd4c5802f219655c46033ed48..a2d0e127b2aec0b876ecbbdd88d5050404f8ff64 100644
--- a/Source/bindings/templates/union.cpp
+++ b/Source/bindings/templates/union.cpp
@@ -133,7 +133,7 @@ void V8{{container.cpp_class}}::toImpl(v8::Isolate* isolate, v8::Handle<v8::Valu
 
     {% else %}
     {# 19. TypeError #}
-    exceptionState.throwTypeError("Not a valid union member.");
+    exceptionState.throwTypeError("The provided value is not of type '{{container.idl_type}}'");
     {% endif %}
 }