“Bad food is bad. Bad books are bad. Bad movies are not always bad,” critic Scott Weinberg tells us in Best Worst Movie, an absorbing and surprisingly well-reported look back at the immortal 1989 trainwreck, Troll 2. It’s directed by Michael Paul Stephenson, who appeared in the film when he was ten years old. Any cinephile knows the truth of Weinberg’s words. It’s mediocrity that’s the real drag — your Bride Wars, your Rush…