fix problematic error handling in todo app backend code

[MagentaManifold]

The response will never be sent if .send is not called on res, resulting in infinite waiting (or timeout).

[MagentaManifold]

I spent an hour debugging my own lab4 code because of this 😅

I haven't checked out example code for future lectures, but please fix them if this error is also in them.

And, please add tests to verify the behavior of 404 responses, if possible, so that this kind of bugs can be eradicated.