Nottingham PlayhouseJenna Russell captures the tragedy in this rags-to-rags story but the power of the songs can't hide the play's weaknesses From the moment Jane Lapotaire stepped out as "this dumpy little woman with a big forehead" more than 40 years ago, Pam Gems' bio-musical about Edith Piaf has been a cronky old vehicle, a low-rent musical towed along by its stars. They are rewarded with a bravura role, which te…