Get HTTP status code error handling

to improve error handling in the HttpError class, it would be useful to have a simpler way to get the request status code, as a `status code` property in the class.

eg.
```python
try:
    self.client.snapshots().get(
        project=[project name],
        snapshot=[description]
    ).execute()
except HttpError as err:
    if err.status_code == 404:
        do_something()
```
