Curtis Sliwa ran a heck of a mayoral race. But face it: He was never going to win. Everyone knew that — even Sliwa. The Guardian Angels founder was on the Republican ballot because no one else wanted to run. No one serious anyway. Failing to find another candidate turned out to be a mistake for the GOP. A big one. In a three-way race no one could have expected, Republicans had a crack at recapturing City Hall. Didn’t happen. The party was caught off guard. Republicans blew an opportunity that