By Steve Holland CHARLESTON, S.C. (Reuters) – Before Donald Trump's put-downs and the stumbles in presidential debates, and before the profound frustration of voters became so unmistakably apparent, Jeb Bush appeared to be best positioned to win back the White House for Republicans in 2016. "I firmly believe the American people must entrust this office to someone who understands that whoever holds it is the servant, not the master, someone who will commit to that service with honor and decency," he said.     How Bush found himself out of the race after just the third nominating contest of 2016 is a cautionary tale of political miscalculation and strategic errors, according to interviews with a dozen Republican operatives, many with close ties to the Bush campaign and others who worked for the last two Republican presidential nominees, conducted during the last days of his campaign.