Fix http.py: Exception -> exception, lint errors, unit test. (#724)

* Exception -> exception.

* Assign exception = None

* Fix lint errors.

* Fix test_media_file_upload_closes_fd_in__del__
diff --git a/googleapiclient/http.py b/googleapiclient/http.py
index a57f83d..9733aa3 100644
--- a/googleapiclient/http.py
+++ b/googleapiclient/http.py
@@ -149,6 +149,7 @@
   """
   resp = None
   content = None
+  exception = None
   for retry_num in range(num_retries + 1):
     if retry_num > 0:
       # Sleep before retrying.
@@ -156,7 +157,7 @@
       LOGGER.warning(
           'Sleeping %.2f seconds before retry %d of %d for %s: %s %s, after %s',
           sleep_time, retry_num, num_retries, req_type, method, uri,
-          resp.status if resp else Exception)
+          resp.status if resp else exception)
       sleep(sleep_time)
 
     try: