Frequently fired lefty news commentator Don Lemon was caught off guard by his arrest Thursday night at a swanky Los Angeles hotel in connection with his participation in a violent takeover of a Minnesota church service, law enforcement sources tell The Post. Lemon, 59, who was in town covering the Grammy awards, was arrested by Department of Homeland Security and FBI agents in an elevator at the Beverly Hilton and taken into custody. He was brought to the downtown Los Angeles Homeland Security