退款通知邮件中要显示退款原因

同时修正：

金额全部退款后，会自动取消活动，但当活动已经结束的话会抛出异常。

```
Completed 500 Internal Server Error in 717ms

StateMachine::InvalidTransition (Cannot transition status via :cancel from :paid (Reason(s): Status translation missing: zh-CN.activerecord.errors.models.event_order.attributes.status.invalid_transition)):
  app/models/event_order_refund_observer.rb:5:in `after_refund'
  app/controllers/order_refunds_controller.rb:43:in `block in alipay_notify'
  app/controllers/order_refunds_controller.rb:36:in `each'
  app/controllers/order_refunds_controller.rb:36:in `alipay_notify'
```

将 `order.cancel!` 的调用改为 `order.cancel`，不加感叹号。

#579