Fix SchemaEditor.__exit__ to handle exceptions correctly

        if exc_type is None:

            for sql in self.deferred_sql:

                self.execute(sql)

            atomic(self.connection.alias, self.connection.features.can_rollback_ddl).__exit__(None, None, None)

        else:

            # Continue propagating exception

            return None

        atomic(self.connection.alias, self.connection.features.can_rollback_ddl).__exit__(exc_type, exc_value, traceback)

    # Core utility functions