Bugfix: bulletproof unexpected_error_msg at 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 wanted to contribute a rock-solid `unexpected_error_msg` implementation.
Moved import to the top
Merge pull request #8262 from icoloma/patch-1

Bugfix: bulletproof unexpected_error_msg at gce.py