repo_pull: Handle abnormal responses

urllib.error.HTTPError works as a http response object (same type that
urlopen() returns). Handle the exception by reading and parsing the
response body of the error.

Test: ./repo_review.py --add-reviewer [invalid username] [query]
Test: ./repo_review.py --add-reviewer [ambiguous username] [query]
Change-Id: Ibfc261d83f2f9efd7873835f74ce8d5e980821c9

tools/repo_pull/gerrit.py

@@ -26,6 +26,11 @@ import json
 import os
 import sys
 
+try:
+    from urllib.error import HTTPError  # PY3
+except ImportError:
+    from urllib2 import HTTPError  # PY2
+
 try:
     from urllib.request import (
         HTTPBasicAuthHandler, Request, build_opener)  # PY3
@@ -146,16 +151,19 @@ def _make_json_post_request(url_opener, url, data, method='POST'):
 
     request = Request(url, data, headers)
     request.get_method = lambda: method
-    response_file = url_opener.open(request)
+
     try:
+        response_file = url_opener.open(request)
+    except HTTPError as error:
+        response_file = error
+
+    with response_file:
         res_code = response_file.getcode()
         # Nothing to parse if response is '204 No Content'
         if res_code == 204:
             return (res_code, None)
         res_json = _decode_xssi_json(response_file.read())
         return (res_code, res_json)
-    finally:
-        response_file.close()
 
 
 def set_review(url_opener, gerrit_url, change_id, labels, message):