Fountain parser: scenes without scene numbers cause problems

[audionerd]

Fountain scripts that contain no scene definition, only actions, will be parsed into scenes without scene numbers. Storyboarder doesn't realize this until after it loads the Fountain file, and it can't find the .storyboarder scene file for the first scene, and then it throws a (somewhat confusing) error.

We could probably find a better way to warn users if their Fountain script has no numbered scenes.