What was the Government's argument regarding Mr. Koch's preexisting conditions, including how this should reduce the damages awarded to Mr. Koch? What were the two exceptions to the general rule regarding a plaintiff's preexisting condition that were cited from Maurer? Why did the Government contend that Maurer was inapplicable? How did the Court reference the Restatement of Torts when responding to the Government's argument concerning Maurer? What was the Court's ultimate holding in this case