fixed error messages for denied attribute access

I was getting obscure `[string "<python>"]:4: error during Python str() call` errors. The real error was a denied attribute access caused by custom `attribute_handlers`: `attr_getter` raises an AttributeError. 

What I haven't yet understood is why aren't AttributeErrors reraised in my case. In simple examples they are reraised. The code is quite hairy, it uses coroutines and throws Python exceptions. I haven't extracted a small test case from it yet. Any ideas? 

But it seems that error messages worth fixing anyways.