(Reuters) - Prime Minister David Cameron's handling of an expenses scandal engulfing one of his ministers has put him under pressure before European elections, threatening his Conservative party's already clouded electoral outlook. Cameron, whose party risks being beaten into third place in next month's European Parliament elections behind the anti-EU UK Independence party (UKIP), has stood by Maria Miller, Britain's minister for culture, since a parliamentary report ordered her to pay back wrongly claimed expenses last week. But the media and public backlash against Miller shows no sign of abating despite Cameron's efforts to end it. Many of his own...