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-r2dbc/src/main/java/org/springframework/r2dbc/connection/R2dbcTransactionManager.java

@@ -228,8 +228,8 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 									.then(Mono.error(ex));
 						}
 						return Mono.error(ex);
-					})).onErrorResume(ex -> Mono.error(new CannotCreateTransactionException(
-							"Could not open R2DBC Connection for transaction", ex)));
+					})).onErrorMap(ex -> new CannotCreateTransactionException(
+							"Could not open R2DBC Connection for transaction", ex));
 		}).then();
 	}
 

spring-r2dbc/src/test/java/org/springframework/r2dbc/core/AbstractDatabaseClientIntegrationTests.java

@@ -50,7 +50,7 @@ abstract class AbstractDatabaseClientIntegrationTests {
 		Mono.from(connectionFactory.create())
 				.flatMapMany(connection -> Flux.from(connection.createStatement("DROP TABLE legoset").execute())
 						.flatMap(Result::getRowsUpdated)
-						.onErrorResume(e -> Mono.empty())
+						.onErrorComplete()
 						.thenMany(connection.createStatement(getCreateTableStatement()).execute())
 						.flatMap(Result::getRowsUpdated).thenMany(connection.close())).as(StepVerifier::create)
 				.verifyComplete();

spring-r2dbc/src/test/java/org/springframework/r2dbc/core/AbstractTransactionalDatabaseClientIntegrationTests.java

@@ -69,7 +69,7 @@ abstract class AbstractTransactionalDatabaseClientIntegrationTests  {
 		Mono.from(connectionFactory.create())
 				.flatMapMany(connection -> Flux.from(connection.createStatement("DROP TABLE legoset").execute())
 						.flatMap(Result::getRowsUpdated)
-						.onErrorResume(e -> Mono.empty())
+						.onErrorComplete()
 						.thenMany(connection.createStatement(getCreateTableStatement()).execute())
 						.flatMap(Result::getRowsUpdated).thenMany(connection.close())).as(StepVerifier::create).verifyComplete();