`session.destroy(error)` should throw if there's no `error` event handler

[szmarczak]

This is because the Agent class attaches its own error handler.

It should check if someone has attached an additional handler. If they did, don't throw. If they didn't, throw.

session.on('error', error => {
    ...

    if (listenerCount === 0) {
        throw error;
    }
});