Pre-defined error responses for known HTTP errors

Things like 400, 404, 5xx can have a pre-defined set of functions that return an elegant response:

``` php
Api::errorNotFound(); // 404 
Api::errorBadRequest(); // 400
Api::errorInternal(); // 500
Api::errorForbidden(); // 403
Api::errorUnauthorized(); // 401
```

etc.
