[讨论] FlowException 和 DegradeException 的 detailMessage 为 null，是否可以适当优化

## Issue Description

Type: *feature request*
**Sentinel 版本** : 2.0.0-alpha
**问题**：抛出的FlowException和DegradeException中，detailMessage为null，这里能够提供一些更有价值的信息，例如将resource、rule的摘要等填入其中

### Describe what feature you want
完善FlowException和DegradeException的detailMessage

社区内是否对此有相关规划？

Start by locating the FlowException and DegradeException definitions and the code that constructs or throws them. Review how resource and rule information is available at those points, then check the issue discussion for the intended message content. Done means both exceptions provide agreed, useful detailMessage values without losing existing behavior.

Written by the indexing model from the issue text.