minor usability issue; scheme does not contain the required palette properties

[clach04]

https://github.com/tinted-theming/tinted-builder-rust/releases/tag/v0.13.2 will fail with:

Error: Unable to deserialize scheme ".......yaml": base24 scheme does not contain the required palette properties

But it doesn't print out what's missing.

To reproduce, edit a working color scheme, delete or comment out (say), base0F.