'Mismatched type' error doesn't use aliased type names

``` rust
type mytype = f32;

fn main() {
    let foo: mytype = 1usize;
}
```

fails with:

>  expected `f32`,
>    found `usize`

but I'd expect it to say something along the lines:

>  expected `mytype` (`f32`)
>    found `usize`

(The problem has been mentioned in comments of #3417)
