After executing the file, the program will ask for a valid file from where it will load the graph. For wrong input, 
it will show an error message ("Invalid File")

After you enter a valid file, then the program will ask for a source and a destinantion. If source or the destination is 
invalid, then it will show an error message ("Unknown source or destination")

If the source and destination both are valid and there is a path between them, then it will show the primary graph. After
you close the window of the primary graph, the optimal path between source and destination will be shown.

If there is no path between source and destination then the primary graph will be shown, indicating source and destination.
Also, a message will be printed in the console, ("Path not found")