Give Eric Idle credit for honesty. The comedian calls his tour "Eric Idle Exploits Monty Python: A Rather Stupid Evening of Skits and Songs." Welcoming the capacity crowd at the Warner Theatre yesterday to the first night of a two-night stand, Idle explained that he was taking advantage of not only his friends and partners in his former comedy troupe, but also the audience--which eagerly swore to be "utterly exploited."