Simplify amending message on job result error

All GoogleAPICallError (GoogleCloudError) instances have the "message"
attribute, even in Python 3, thus the additional info can be appended
directly to that attribute.

bigquery/google/cloud/bigquery/job.py

@@ -2913,9 +2913,8 @@ def result(self, timeout=None, page_size=None, retry=DEFAULT_RETRY):
                     self.job_id, retry, project=self.project, location=self.location
                 )
         except exceptions.GoogleCloudError as exc:
-            new_msg = str(exc) + self._format_for_exception(self.query, self.job_id)
-            new_exc = type(exc)(new_msg)
-            raise six.raise_from(new_exc, exc)
+            exc.message += self._format_for_exception(self.query, self.job_id)
+            raise
 
         # If the query job is complete but there are no query results, this was
         # special job, such as a DDL query. Return an empty result set to