Invalid default-theme and preferred-dark-theme now cause an error

[hollow-owl]

This causes building to fail if default-theme or preferred-dark-theme is not one of the built-in themes.

If book.toml has an invalid default-theme:

mdbook build outputs an error saying why:

 INFO Book building has started
 INFO Running the html backend
ERROR Failed to deserialize `output.html`
        Caused by: unknown variant `does-not-exist`, expected one of `ayu`, `coal`, `light`, `navy`, `rust`
in `default-theme`

 INFO HTML book written to `/scratch/book`

Fixes #3140