adding a constant for exception message

Author: Matt

src/main/java/io/github/makbn/jlmap/exception/JLMapNotReadyException.java

@@ -6,12 +6,13 @@
  * JLMap Exception which is thrown when changing the map before it gets ready!
  * Leaflet map gets fully ready after the {{@link javafx.concurrent.Worker.State}} of {{@link javafx.scene.web.WebEngine}}
  * changes to {{@link javafx.concurrent.Worker.State#SUCCEEDED}}
- * by: Mehdi Akbarian Rastaghi (@makbn)
+ * @author Mehdi Akbarian Rastaghi (@makbn)
  */
 public class JLMapNotReadyException extends JLException{
+    private static final String MAP_IS_NOT_READY_YET = "Map is not ready yet!";
 
     @Builder
     public JLMapNotReadyException() {
-        super("map is not ready yet!");
+        super(MAP_IS_NOT_READY_YET);
     }
 }