init: enhance error message **psetinvalidn

Add the index to the error message in MPIR_Session_get_nth_pset_impl.

Author: hzhou

src/mpi/errhan/errnames.txt

@@ -10,6 +10,7 @@
 **finalized:MPI has already called MPI_Finalize
 **inittwice:Cannot call MPI_INIT or MPI_INIT_THREAD more than once
 **psetinvalidn:Invalid pset number
+**psetinvalidn %d:Invalid pset number - %d
 **psetinvalidname:Invalid pset name
 **session:Invalid MPI_Session handle
 **sessionnull: NULL MPI_Session

src/mpi/init/init_impl.c

@@ -195,7 +195,8 @@ int MPIR_Session_get_nth_pset_impl(MPIR_Session * session_ptr, MPIR_Info * info_
     }
 
     if (!MPIR_pset_list[i]) {
-        MPIR_ERR_SETANDSTMT(mpi_errno, MPI_ERR_ARG, goto fn_fail, "**psetinvalidn");
+        MPIR_ERR_SETANDSTMT1(mpi_errno, MPI_ERR_ARG, goto fn_fail, "**psetinvalidn",
+                             "**psetinvalidn %d", n);
     }
 
     int len = strlen(MPIR_pset_list[i]);