Spanner: 'test_transaction_batch_update_and_execute_dml' systest flakes with 'None' status code.

[tseaver]

Similar to #7504, but this time the status code is None.

From this CI failure:

_________ TestSessionAPI.test_transaction_batch_update_and_execute_dml _________

self = <tests.system.test_system.TestSessionAPI testMethod=test_transaction_batch_update_and_execute_dml>

 def test_transaction_batch_update_and_execute_dml(self):
 retry = RetryInstanceState(_has_all_ddl)
 retry(self._db.reload)()

 session = self._db.session()
 session.create()
 self.to_delete.append(session)

 with session.batch() as batch:
 batch.delete(self.TABLE, self.ALL)

 insert_statements = list(self._generate_insert_statements())
 update_statements = [
 (
 "UPDATE contacts SET email = @email " "WHERE contact_id = @contact_id;",
 {"contact_id": 1, "email": "phreddy@example.com"},
 {"contact_id": Type(code=INT64), "email": Type(code=STRING)},
 )
 ]

 delete_statement = "DELETE contacts WHERE TRUE;"

 def unit_of_work(transaction, self):
 rows = list(transaction.read(self.TABLE, self.COLUMNS, self.ALL))
 self.assertEqual(rows, [])

 status, row_counts = transaction.batch_update(
 insert_statements + update_statements
 )
 self._check_batch_status(status.code)
 self.assertEqual(len(row_counts), len(insert_statements) + 1)
 for row_count in row_counts:
 self.assertEqual(row_count, 1)

 row_count = transaction.execute_update(delete_statement)

 self.assertEqual(row_count, len(insert_statements))

> session.run_in_transaction(unit_of_work, self)

tests/system/test_system.py:874:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
google/cloud/spanner_v1/session.py:299: in run_in_transaction
 return_value = func(txn, *args, **kw)
tests/system/test_system.py:865: in unit_of_work
 self._check_batch_status(status.code)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

status_code = 10

 @staticmethod
 def _check_batch_status(status_code):
 if status_code != code_pb2.OK:
> raise exceptions.from_grpc_status(status_code, "batch_update failed")
E google.api_core.exceptions.GoogleAPICallError: None batch_update failed

[tseaver]

The status_code isn't actually None, but 10 (for an aborted transaction). However, the call to exceptions.from_grpc_status to convert that status to an exception fails to look up the Aborted class, because it wants an instance of the grpc.StatusCode enum, and so it returns a generic exception, with no status code.