Raise more detailed error message by including the server response sent

Earlier error trace:
```
---------------------------------------------------------------------------
HTTPError                                 Traceback (most recent call last)
<ipython-input-17-31c82c47c84d> in <module>()
----> 1 r2.raise_for_status()

/nail/home/prateek/pg/mypy/lib/python2.6/site-packages/requests/models.pyc in raise_for_status(self)
    793
    794         if http_error_msg:
--> 795             raise HTTPError(http_error_msg, response=self)
    796
    797     def close(self):

HTTPError: 500 Server Error: Internal Server Error
```

Newer error trace: (added **{"reason": "File is not a zip file", "error": "zipfile.BadZipfile"}** )
```
/nail/home/prateek/pg/swagger-py/swaggerpy/response.pyc in result(self, timeout)
     50         response = self._http_client.wait(timeout)
     51         try:
---> 52             response.raise_for_status()
     53         except IOError as e:
     54             args = list(e.args)

/nail/home/prateek/pg/mypy/lib/python2.6/site-packages/requests/models.pyc in raise_for_status(self)
    793
    794         if http_error_msg:
--> 795             raise HTTPError(http_error_msg, response=self)
    796
    797     def close(self):

HTTPError: 500 Server Error: Internal Server Error : {"reason": "File is not a zip file", "error": "zipfile.BadZipfile"}
```