Several type errors incorrectly still have type variables in them

For example:
Compiling

```
fn main() {
    let z = ();
    let y = z[0];
}
```

results in

```
nubs/unboxed_vecs.rs:4:12: 4:15 error: cannot index a value of type `<V1>`
nubs/unboxed_vecs.rs:4     let y = z[0];
```

I think this is because code is calling ty_to_str instead of fcx.infcx.ty_to_str.
