Optimize uses of onErrorResume()

This commit replaces uses of onErrorResume() with
- onErrorMap() in places where onErrorResume() is just used to map to a
  different exception.
- onErrorComplete() where onErrorResume() just maps to Mono.empty().
- onErrorReturn() where onErrorResum() just maps to Mono.just().

Closes gh-31352

spring-web/src/main/java/org/springframework/http/codec/ResourceHttpMessageWriter.java

@@ -191,7 +191,7 @@ public class ResourceHttpMessageWriter implements HttpMessageWriter<Resource> {
 		if (InputStreamResource.class != resource.getClass()) {
 			return Mono.fromCallable(resource::contentLength)
 					.filter(length -> length != -1)
-					.onErrorResume(IOException.class, t -> Mono.empty())
+					.onErrorComplete(IOException.class)
 					.subscribeOn(Schedulers.boundedElastic());
 		}
 		else {

spring-web/src/main/java/org/springframework/web/server/ServerWebExchange.java

@@ -148,10 +148,10 @@ public interface ServerWebExchange {
 	 */
 	default Mono<Void> cleanupMultipart() {
 		return getMultipartData()
-				.onErrorResume(t -> Mono.empty())  // ignore errors reading multipart data
+				.onErrorComplete()  // ignore errors reading multipart data
 				.flatMapIterable(Map::values)
 				.flatMapIterable(Function.identity())
-				.flatMap(part -> part.delete().onErrorResume(ex -> Mono.empty()))
+				.flatMap(part -> part.delete().onErrorComplete())
 				.then();
 	}
 

spring-web/src/main/java/org/springframework/web/server/adapter/DefaultServerWebExchange.java

@@ -249,11 +249,11 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 	public Mono<Void> cleanupMultipart() {
 		if (this.multipartRead) {
 			return getMultipartData()
-					.onErrorResume(t -> Mono.empty()) // ignore errors reading multipart data
+					.onErrorComplete() // ignore errors reading multipart data
 					.flatMapIterable(Map::values)
 					.flatMapIterable(Function.identity())
 					.flatMap(part -> part.delete()
-									.onErrorResume(ex -> Mono.empty()))
+									.onErrorComplete())
 					.then();
 		}
 		else {