Extend RestErrorHandler to catch NestedRuntimeException

see issue #1002 - `RestErrorHandler` should not be limited to `RestClientException`. When there is a network problem (`SocketTimeoutException`, `HttpHostConnectException` or an exception thrown by Jackson) Spring's RestTemplate automatically throws a `ResourceAccessException`. We should be able to handle those exceptions as well. 
`ResourceAccessException` and `RestClientException` are both subclasses of `NestedRuntimeException`.

I have not changed the method name `onRestClientExceptionThrown` in order to ensure backwards compatibility. Current users implementing `RestErrorHandler` should not face any problems, even when catching the exceptions manually prior to handling them, since they are all subclasses of `NestedRuntimeException`.