DSS-3231 Add success return to gs_to_bq transfer method (#31)

This is because it can cause silent failures without it. There is a
try-except (without a 'raise') within the load_table_from_uris method,
so if there is an error in the BigQuery load job, the try-except causes
it to appear as a success. Hence we can't then ascertain whether the job
failed and raise an error accordingly.

See JIRA ticket comment
[here](https://timeoutgroup.atlassian.net/browse/DSS-3231?focusedCommentId=263012)
for local testing of this method (using a copy of the transfer module)

Author: JHerman2004

to_data_library/data/transfer.py

@@ -183,7 +183,7 @@ def gs_to_bq(self, gs_uris, table,
 
         try:
             # Start the load job
-            bq_client.load_table_from_uris(
+            return bq_client.load_table_from_uris(
                 gs_uris, table_ref, job_config=job_config
             )