Visual diffs succeed if new component reference images weren't committed

[swese44]

Basically builds will pass if a new component is added without generating and committing the visual diffs. example:
#321

this is because new images are auto-generated if they're missing, and if somebody doesn't manually add them then the CI run will auto-generate them and succeed.

i think we should update https://github.com/unindented/react-styleguidist-visual to fail if it generates new images to force us to manually create those images