Adapting a book to the screen is a dicey affair, and the task is particularly daunting with a bestseller like John Green’s 2012 novel, “The Fault in Our Stars.” Fortunately for fans, director Josh Boone skillfully dodges the pitfalls that have caused so many literary works to fail in theaters. Boone’s “Fault in Our Stars” is, like the book, a tearjerker, but it is also thoughtful, sweet and often funny.