Make ResumableUploadError derive from HttpError.

Fixes issue #242.

Reviewed in https://codereview.appspot.com/7415048/.
diff --git a/tests/test_discovery.py b/tests/test_discovery.py
index ac6f97c..92213ac 100644
--- a/tests/test_discovery.py
+++ b/tests/test_discovery.py
@@ -769,7 +769,11 @@
         'location': 'http://upload.example.com'}, ''),
       ])
 
-    self.assertRaises(ResumableUploadError, request.execute, http=http)
+    try:
+      request.execute(http=http)
+      self.fail('Should have raised ResumableUploadError.')
+    except ResumableUploadError, e:
+      self.assertEqual(400, e.resp.status)
 
   def test_resumable_media_fail_unknown_response_code_subsequent_request(self):
     """Not a multipart upload."""