opt-in for request/response info in HTTPStatusError message

Allow users to restore old error message behavior (removed in #219 9ac6dda27ee0665c0c956055f0ddd221ba79104c) by choosing to
display the request and/or the response.

```rb
Excon::Errors::HTTPStatusError.debug_request = true
Excon::Errors::HTTPStatusError.debug_response = true
```