Add ClientResponse::createError

This commit introduces ClientResponse::createError, returning
a Mono that terminates with a WebClientException.

Closes gh-27637

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/ClientResponse.java

@@ -181,6 +181,18 @@ public interface ClientResponse {
 	 */
 	Mono<WebClientResponseException> createException();
 
+	/**
+	 * Create a {@code Mono} that terminates with a
+	 * {@link WebClientResponseException}, containing the response status,
+	 * headers, body, and the originating request.
+	 * @param <T> the reified type
+	 * @return a {@code Mono} that fails with a
+	 * {@link WebClientResponseException}.
+	 * @see #createException()
+	 * @since 6.0
+	 */
+	<T> Mono<T> createError();
+
 	/**
 	 * Return a log message prefix to use to correlate messages for this exchange.
 	 * <p>The prefix is based on {@linkplain ClientRequest#logPrefix()}, which

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponse.java

@@ -223,6 +223,11 @@ class DefaultClientResponse implements ClientResponse {
 				});
 	}
 
+	@Override
+	public <T> Mono<T> createError() {
+		return createException().flatMap(Mono::error);
+	}
+
 	@Override
 	public String logPrefix() {
 		return this.logPrefix;

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/support/ClientResponseWrapper.java

@@ -153,6 +153,11 @@ public class ClientResponseWrapper implements ClientResponse {
 		return this.delegate.createException();
 	}
 
+	@Override
+	public <T> Mono<T> createError() {
+		return this.delegate.createError();
+	}
+
 	@Override
 	public String logPrefix() {
 		return this.delegate.logPrefix();

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientResponseTests.java

@@ -28,6 +28,7 @@ import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
 
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.core.codec.ByteArrayDecoder;
@@ -353,6 +354,35 @@ public class DefaultClientResponseTests {
 		assertThat(exception.getResponseBodyAsByteArray()).isEqualTo(bytes);
 	}
 
+	@Test
+	public void createError() {
+		byte[] bytes = "foo".getBytes(StandardCharsets.UTF_8);
+		DefaultDataBuffer dataBuffer = DefaultDataBufferFactory.sharedInstance.wrap(ByteBuffer.wrap(bytes));
+		Flux<DataBuffer> body = Flux.just(dataBuffer);
+		httpHeaders.setContentType(MediaType.TEXT_PLAIN);
+		given(mockResponse.getStatusCode()).willReturn(HttpStatus.NOT_FOUND);
+		given(mockResponse.getRawStatusCode()).willReturn(HttpStatus.NOT_FOUND.value());
+		given(mockResponse.getBody()).willReturn(body);
+
+		List<HttpMessageReader<?>> messageReaders = Collections.singletonList(
+				new DecoderHttpMessageReader<>(new ByteArrayDecoder()));
+		given(mockExchangeStrategies.messageReaders()).willReturn(messageReaders);
+
+		Mono<String> resultMono = defaultClientResponse.createError();
+		StepVerifier.create(resultMono)
+				.consumeErrorWith(t -> {
+					assertThat(t).isInstanceOf(WebClientResponseException.class);
+					WebClientResponseException exception = (WebClientResponseException) t;
+					assertThat(exception.getStatusCode()).isEqualTo(HttpStatus.NOT_FOUND);
+					assertThat(exception.getMessage()).isEqualTo("404 Not Found");
+					assertThat(exception.getHeaders()).containsExactly(entry("Content-Type",
+							Collections.singletonList("text/plain")));
+					assertThat(exception.getResponseBodyAsByteArray()).isEqualTo(bytes);
+
+				})
+				.verify();
+	}
+
 
 	private void mockTextPlainResponse(Flux<DataBuffer> body) {
 		httpHeaders.setContentType(MediaType.TEXT_PLAIN);