Emit a better warning when default-exporting an inappropriate declaration

Courtesy of @LPGhatguy in #3914.

Instead of going off the rails, we should resiliently parse the following:

``` TypeScript
export default interface User {
    hi: string;
}
```

And give an error like

```
An 'interface' declaration cannot be 'export default'. Consider writing 'export default User;' separately.
```
