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/test/java/org/springframework/http/server/reactive/ServerHttpResponseTests.java

@@ -105,7 +105,7 @@ class ServerHttpResponseTests {
 		headers.setContentType(MediaType.APPLICATION_JSON);
 		headers.set(HttpHeaders.CONTENT_ENCODING, "gzip");
 		headers.setContentLength(12);
-		response.writeWith(body).onErrorResume(ex -> Mono.empty()).block();
+		response.writeWith(body).onErrorComplete().block();
 
 		assertThat(response.statusCodeWritten).isFalse();
 		assertThat(response.headersWritten).isFalse();