Refactor apitools legacy 'handle_http_exceptions' for sanity.

The [apitools code which handles the various errors being raised when making a request](https://github.com/tseaver/gcloud-python/blob/streaming-strip_unused_features/gcloud/streaming/http_wrapper.py#L225-L276) is seriously awkward, and likely a bug magnet.
