Add custom ResourceNotFoundException class

Author: codesungrape

src/main/java/com/codesungrape/hmcts/bookapi/exception/ResourceNotFoundException.java

@@ -0,0 +1,13 @@
+package com.codesungrape.hmcts.bookapi.exception;
+
+/**
+ * Custom exception to signal that a requested resource (Book, Reservation, etc.)
+ * could not be found, typically mapping to HTTP 404.
+ */
+
+public class ResourceNotFoundException extends RuntimeException {
+
+    public ResourceNotFoundException(String message) {
+        super(message);
+    }
+}