It’s tough to pinpoint the moment Eve Ensler’s play — a study of war-ravaged Bosnian women — veers off course. Perhaps it’s when someone compares a sardine to a refugee. Or when others offer that Americans are Big Mac-eatin’, therapy-dependent richies. Or when the girls get drunk and dance around like they’re in some Hollywood chick flick. Probably not what you’d expect from Ensler, the playwright/activist who raised eyebrows and awareness with 1999’s The Vagina Monologues. There’s powerful stuff: a mother (an excellent Mirjana Jokovic) who lost — literally lost — her child while fleeing; a doctor stripped of her craft and identity (Diane Venora, intensity personified). The aim is admirable; Necessary just misses its target.