Auto merge of #144 - yvan-sraka:patch-0, r=komaeda

Add errors to exercises that compile without user changes

Hi !

I played a bit with rustlings, and I felt that some exercises were incorrect because they passed the tests without me needing to edit the files!

This gave me the feeling that the exercise was skiped! Especially when I use `rustlings watch`, it is easy to miss an exercise because the compilation error that is displayed is the one of the next exercise ...

It is easy to identify "broken" exercises with:

```bash
% find exercises -name "*.rs" | xargs -n 1 rustlings run
...
✅ Successfully ran exercises/move_semantics/move_semantics4.rs
✅ Successfully tested exercises/test2.rs
```

My suggestion is to make sure that these files trigger a compilation error by adding a simple syntax error (e.g. with `???` in the code that must change) so that our Rustacean can then play with it!

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