error handling improvements

As of version 2.02 pgn4web implements an improved error handling.

Rather than displaying popup messages, errors alert messages are now logged, such as failure to load PGN data, incorrect PGN games or incorrect FEN strings. When an error is encountered, the top left chessboard square (A8 assuming White on bottom, H1 otherwise) will flash to signal the exception. The error alert log can be reviewed clicking on the same top left chessboard square.

The debugging feature can be used both while developing new HTML pages and when entering PGN data using any of the pgn4web tools, for instance when adding a chessboard to a wordpress based site using the embed-chessboard plugin.