This commit changes the WebClient so that it now throws a
`WebClientResponseException` for `ResponseSpec.bodyTo`. This newly
introduces exception contains the status code, headers, and body of the
response message.
As a consequence of the above, we had to change `onStatus` so that the
`exceptionFunction` now returns a `Mono<Throwable>` rather than a
`Throwable`, which it was before. The Mono allows for asynchronous
operations, such as reading the contents of the body.
Issue: SPR-15824
5394cc0c63