Bugfix: bulletproof unexpected_error_msg for gce.py

This method was still failing for me with a "missing http_code" message. After applying this change, the error message is:

```
msg: Unexpected response: {'value': 'PyCrypto library required for Service Account Authentication.'}
```

I guess a bulletproof error message function was needed after all.

Please excuse the errors in style, I don't code in Python regularly.