Error message for n_divertors != 1,2

Found this in #4522 

<img width="778" height="151" alt="Image" src="https://github.com/user-attachments/assets/7930e596-117e-4f6c-9221-5835307fa201" />

This should be removed and a check for n_divertors done beforehand - currently there is a check in `blkt_outboard_poloidal_plasma_angle()` but this fn is only called in dcll and hcpb 