code: protected ResponseErrorHandler buildServerErrorOnlyResponseErrorHandler(){
  		return new DefaultResponseErrorHandler(){
  			@Override
  			protected boolean hasError(HttpStatus statusCode) {
  				return statusCode.series() == HttpStatus.Series.SERVER_ERROR;
  			}
  		};
  	}
docs: Create a customized ResponseErrorHandler that ignores HttpStatus.Series.CLIENT_ERROR.
  That means responses with status codes like 400/404/401/403/etc are not treated as error, therefore no exception will be thrown in those cases.
  Responses with status codes like 500/503/etc will still cause exceptions to be thrown.
  @return the ResponseErrorHandler that cares only HttpStatus.Series.SERVER_ERROR