Well obviously "if your code does the right thing" then it's going to be secure rather than insecure... by definition. That's again a pretty unhelpful tautology.

I'm asking about the code in the webpage, not code I'm writing personally. I'm saying let's assume it has error-handling added to it. That's it. I am not writing any code otherwise. Is that code doing "the right thing"? Or are there more things it needs to be doing?