Provide response body for status code 400

## 🚀 Feature Requests

Fetch and provide response body for certain error status codes.

### 🎤 Pitch

Many APIs provide further information in the response on bad client requests (status code 400)
This allows to forward that information to the user in order to explain what he did wrong.

Sadly, since flutter_cache_manager handles all errors equally by discarding the response contents, that information is lost, only allowing us to show a generic "invalid request" message.

未指定文件或测试。首先追踪 flutter_cache_manager 如何处理 HTTP 错误并丢弃响应内容；使用包含正文的 400 响应验证该行为，并考虑让调用方能够访问这些信息而不只是获得通用错误所需的工作。

由索引模型根据 Issue 内容生成。