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.