prevent future failures of bulkdata_update.q/topic_mapping_reducer.py

Summary:
Intended to fix failures such as in job j-2QQ7AT0GCK7O1 like

Traceback (most recent call last):
  File "/mnt/var/lib/hadoop/mapred/taskTracker/hadoop/jobcache/job_201307101525_0016/attempt_201307101525_0016_r_000000_2/work/./topic_mapping_reducer.py", line 50, in <module>
    main()
  File "/mnt/var/lib/hadoop/mapred/taskTracker/hadoop/jobcache/job_201307101525_0016/attempt_201307101525_0016_r_000000_2/work/./topic_mapping_reducer.py", line 37, in main
    ancestor_titles.append(topics_map[ancestor_key]['title'])
KeyError: u'ag5zfmtoYW4tYWNhZGVteXJmCxIFVG9waWMiKFhYSkp5ZlhONE1fZ05DM3ItRUF0dEZZeDRDY1ItaE5KNjZBOXYxc3QMCxIFVG9waWMiKHpmd3hqU2htSjFLSmF1a01IZm5odERpcVUyYjV5NEs4bDFGeXdiaTAM'

Test Plan:
Uploaded to S3, and ran the following section of bulkdata_update.q successfully.

SET mapred.reduce.tasks=1;
FROM (
  SELECT key, json FROM Topic
  DISTRIBUTE BY key
) map_out
INSERT OVERWRITE TABLE topic_mapping
SELECT TRANSFORM(map_out.key, map_out.json)
USING 's3://ka-mapreduce/code/py/topic_mapping_reducer.py'
AS topic_key, topic_title, ancestor_keys_json, ancestor_titles_json;
SET mapred.reduce.tasks=-1;

Reviewers: sitanchen

Reviewed By: sitanchen

Differential Revision: http://phabricator.khanacademy.org/D2978