[#144] ResponseRethrowingErrorHandler fails in getBody() when handling exceptions

I managed to reproduce bug in #144 in test. Similar fix compared to [`DefaultResponseErrorHandler`](https://github.com/spring-projects/spring-framework/blob/master/spring-web/src/main/java/org/springframework/web/client/DefaultResponseErrorHandler.java#L107-L109), but I don't silently ignore `IOException`.