Prior to this commit, the wrong `HandlerResultHandler` could be
resolved when handling exceptions; this could happen only if the
original handler and exception handler had different signatures:
```
Publisher<String> originalHandler() { ... }
@ExceptionHandler(MyCustomException.class)
ResponseEntity<Mono<String>> handleException() { ... }
```
In that case, the `ResponseBodyResultHandler` would be used when
handling exceptions instead of the `ResponseEntityResultHandler`.
This commit ensures that the `HandlerResult` returned by the exception
handler is used to resolve the `HandlerResultHandler`. The latter will
process the result and use it to write to the HTTP response.
Issue: SPR-14876
95abd18fea