Fix for perm_lst error

These commits fix the TypeError encountered in the random_state.permutation call used to generate perm_lst in the sampling method of StanModel.  It converts n_kept to an integer, the type required by random_state.permutation.  

Additionally, it has small commits that add informative error messages related to user-input problems I encountered in my initial tests with Pystan.  I hope these messages would help others diagnose problems in their code more quickly.