build: west: Update the error message when not in west dir

Zephyr verifies if it is built from a west-initialized directory.
The message changed in west@c08061cef1c7b0e19a58a82db731098e2f4bba4a.

Closes #18034.

Signed-off-by: Piotr Zierhoffer <pzierhoffer@antmicro.com>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>