“While reference has been made above to some of the rules which should be observed in a well-conducted trial to safeguard the fairness of the proceedings, it is not every departure from good practice which renders the trial unfair … But the right of a criminal defendant to a fair trial is absolute. There will come a point when the departure from good practice is so gross, or so persistent, or so irremediable that an appellate court will have no choice but to condemn a trial as unfair and quash a conviction as unsafe, however strong the grounds for believing the defendant to be guilty.”