Tests: make test_pull_image_exception reasonable
Tests: make test_pull_image_exception reasonable

The original way raising an errors.APIError won't execute the
code `except Exception e`, as the tested code is like this:

```python
except errors.APIError as api_error:
    raise exception.ZunException(str(api_error))
except Exception as e:
    msg = _('Cannot download image from docker: {0}')
    raise exception.ZunException(msg.format(e))
```

Change-Id: I95a1a650bc611f61ac03eb5fdf08481c67bb9ab7

kevin-zhaoshuai/zun,kevin-zhaoshuai/zun,kevin-zhaoshuai/zun