Add debug logs to HTML printing

[tresf]

JavaFX allows a few way to attach a console to the JavaFX instance.

https://stackoverflow.com/a/49077436/3196753

The console can be helpful for troubleshooting non-obvious JavaFX engine errors.

WebConsoleListener.setDefaultListener((webView, message, lineNumber, sourceId) -> log.debug("JavaFX says: {} [at {}]", message, lineNumber));