Improve warning message if only one cell is perturbed in scperturbR

[stefanpeidli]

"In case the perturbation status is only found in 1 cell, scPerturbR does not give you an error message about the number, but rather it complains

Error in res$p.value : $ operator is invalid for atomic vectors

It might be nice to raise an error here so people understand what is going wrong. Thanks Annique for finding this.