Add error message to write_simulated_image_stack_from_starfile

[mjo22]

This function takes in a function that generates an image either from 1) RelionParticleStack or 2) RelionParticleStack + RNG key. Whether or not the code tries to do 1) vs 2) depends on seed=None or not. I think this is in the documentation, but could we add a helpful error message to make this clear?