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.

No files or tests are named. Start by tracing how flutter_cache_manager handles HTTP errors and discards response contents; verify the behavior with a 400 response that contains a body, and consider the work done when callers can access that information instead of only a generic error.

索引モデルが issue の本文から書いたものです。