"Cannot use experimental feature" should have an error code

### HHVM Version

3.15, 3.16, latest nightly (2016-12-04)

### Standalone code, or other way to reproduce the problem

In non-pseudomain code:

```Hack
$x = vec[1, 2, 3];
```

`hh_client`

### Expected result

Typechecker error with error code.

### Actual result

```
TypeStructureTest.php:270:8,10: Cannot use experimental feature: vec (Other[0000])
```

0000 is the problem (though it is compatible with HH_IGNORE_ERROR still)