feat/Created new NotificationNotFoundException class

src/main/java/no/ntnu/idi/stud/savingsapp/exception/notification/NotificationNotFoundException.java

+package no.ntnu.idi.stud.savingsapp.exception.notification;
+
+/**
+ * Exception thrown when attempting to retrieve a Notification that does not exist.
+ */
+public class NotificationNotFoundException extends RuntimeException {
+  /**
+   * Constructs a NotificationNotFoundException with the default message.
+   */
+  public NotificationNotFoundException() {
+    super("Notification is not found");
+  }
+}