Update Exception API

- Introduce a DuplicateOrderLineException to signal whenever somebody
wants to add the same product twice instead of using the increment
command.
- Introduce OrderAlreadyConfirmedException to signal whenever the order
is confirmed, so that adding products and incrementing/decrementing
their count is not possible once the order has been confirmed

#BAEL-4767

axon/src/main/java/com/baeldung/axon/coreapi/exceptions/DuplicateOrderLineException.java

@@ -0,0 +1,8 @@
+package com.baeldung.axon.coreapi.exceptions;
+
+public class DuplicateOrderLineException extends IllegalStateException {
+
+    public DuplicateOrderLineException(String productId) {
+        super("Cannot duplicate order line for product identifier [" + productId + "]");
+    }
+}

axon/src/main/java/com/baeldung/axon/coreapi/exceptions/OrderAlreadyConfirmedException.java

@@ -0,0 +1,8 @@
+package com.baeldung.axon.coreapi.exceptions;
+
+public class OrderAlreadyConfirmedException extends IllegalStateException {
+
+    public OrderAlreadyConfirmedException(String orderId) {
+        super("Cannot perform operation because order [" + orderId + "] is already confirmed.");
+    }
+}