Implement useful error message for original cookiecutter command

[DragaDoncila]

Once this repo is renamed and the napari-plugin-template is live, we should implement a meaningful error message for when someone tries to run the old command, referencing the cookiecutter-napari-plugin repo.

Perhaps we remake the cookiecutter repo after renaming this one, and have it throw a meaningful error, or perhaps we rely on GitHub's redirect and just add the dummy cookiecutter config with the error message?

[GenevieveBuckley]

Perhaps we remake the cookiecutter repo after renaming this one, and have it throw a meaningful error, or perhaps we rely on GitHub's redirect and just add the dummy cookiecutter config with the error message?

Yeah, maybe. We'll have to check what the easiest way to generate a good error message with cookiecutter is