Filename to save kernel stderr output.

def stderr_file(self):
        """Filename to save kernel stderr output."""
        stderr_file = None
        if self.connection_file is not None:
            stderr_file = self.kernel_id + '.stderr'
            if self.stderr_dir is not None:
                stderr_file = osp.join(self.stderr_dir, stderr_file)
            else:
                try:
                    stderr_file = osp.join(get_temp_dir(), stderr_file)
                except (IOError, OSError):
                    stderr_file = None
        return stderr_file