#1967 - Improve how graphics::generate handles missing/bad images.

- Made missing_photo match the image format (jpg, png, etc.).
- Added missing_photo.jpg and missing_photo.gif to help with above.
- Improved error messages to user when things go wrong.
- Ensured that missing image placeholders are always copied when there's an error.
- Ensured we don't mistake no file output for a correct file output (delete target before attempt).
- Restructured graphics::generate a bit to work better with above changes.
- Added unit tests for graphics::generate.