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();
 	}