Last week’s prime-time special, introducing viewers to the casting and rehearsal process in bringing this Rodgers and Hammerstein classic to television, did a great job of promotion, stressing the point of just how high-risk, and exciting, a live TV premiere is bound to be. And here it comes. At one point in TV’s history, everything was presented this way – but tonight, a live musical, starring Carrie Underwood and featuring Audra McDonald and Laura Benanti, is an absolute rari