Add detail to error message

src/main/java/GraphDataObjectReader.java

@@ -21,6 +21,7 @@ public class GraphDataObjectReader {
             }
             data = (GraphWithMapData) in.readObject();
         } catch (FileNotFoundException | ClassNotFoundException ex) {
+            System.out.println("Graph/data file " + filename + " not found.");
             ex.printStackTrace();
         }
     }