patch 9.0.0345: error message for list argument could be clearer

Problem:    Error message for list argument could be clearer.
Solution:   Include the argument number. (Yegappan Lakshmanan, closes #11027)

src/typval.c

@@ -508,6 +508,23 @@ check_for_list_arg(typval_T *args, int idx)
     return OK;
 }
 
+/*
+ * Give an error and return FAIL unless "args[idx]" is a non-NULL list.
+ */
+    int
+check_for_nonnull_list_arg(typval_T *args, int idx)
+{
+    if (check_for_list_arg(args, idx) == FAIL)
+	return FAIL;
+
+    if (args[idx].vval.v_list == NULL)
+    {
+	semsg(_(e_non_null_list_required_for_argument_nr), idx + 1);
+	return FAIL;
+    }
+    return OK;
+}
+
 /*
  * Check for an optional list argument at 'idx'
  */