'SailthruResponseError' object has no attribute 'msg'

Hi there,

`reponse.get_error()` only returns error code, but not the error message. However the error message does come through `reponse.get_body()`.

```
>>> response.get_error().code
99
>>> response.get_error().msg
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'SailthruResponseError' object has no attribute 'msg'
```