Add DukeException.java

src/main/java/DukeException.java

@@ -0,0 +1,14 @@
+public class DukeException extends Exception {
+    protected String errormessage;
+
+    public DukeException(String errormessage) {
+        this.errormessage = errormessage;
+    }
+
+    @Override
+    public String toString() {
+        return "____________________________________________________________\n"
+                + this.errormessage
+                + "\n____________________________________________________________";
+    }
+}