Polish "Logback StructuredLogFormatter exceptions are not visible to the user"

See gh-43575

Author: mhalbritter

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/logback/FilteringStatusListener.java

@@ -27,7 +27,7 @@
  *
  * @author Dmytro Nosan
  */
-final class FilteringStatusListener extends ContextAwareBase implements StatusListener, LifeCycle {
+class FilteringStatusListener extends ContextAwareBase implements StatusListener, LifeCycle {
 
 	private final StatusListener delegate;
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java

@@ -495,8 +495,10 @@ private void addOnErrorConsoleStatusListener(LoggerContext context) {
 		FilteringStatusListener listener = new FilteringStatusListener(new OnErrorConsoleStatusListener(),
 				Status.ERROR);
 		listener.setContext(context);
-		context.getStatusManager().add(listener);
-		listener.start();
+		boolean effectivelyAdded = context.getStatusManager().add(listener);
+		if (effectivelyAdded) {
+			listener.start();
+		}
 	}
 
 	void setStatusPrinterStream(PrintStream stream) {