Make ResumableUploadError derive from HttpError.

Fixes issue #242.

Reviewed in https://codereview.appspot.com/7415048/.
diff --git a/apiclient/http.py b/apiclient/http.py
index bfb7f77..a956477 100644
--- a/apiclient/http.py
+++ b/apiclient/http.py
@@ -744,7 +744,7 @@
       if resp.status == 200 and 'location' in resp:
         self.resumable_uri = resp['location']
       else:
-        raise ResumableUploadError("Failed to retrieve starting URI.")
+        raise ResumableUploadError(resp, content)
     elif self._in_error_state:
       # If we are in an error state then query the server for current state of
       # the upload by sending an empty PUT and reading the 'range' header in