set CHANGE make ly_set* functions return LY_ERR values

unify ly_set* function that were returning -1 as error indicator
combined with positive value indicating index inside the set. The most
of other libyang functions return LY_ERR to indicate result. The problem
of these functions was also the limited range of the returned index.
While internally index is uint32_t, it was limited only to a positive
part of int type.
diff --git a/src/parser.c b/src/parser.c
index f1fd349..2a5c7eb 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -414,7 +414,7 @@
     ret = lyd_create_term(schema, value, value_len, dynamic, value_hints, format, prefix_data, node);
     if (ret == LY_EINCOMPLETE) {
         if (!(lydctx->parse_options & LYD_PARSE_ONLY)) {
-            ly_set_add(&lydctx->unres_node_type, *node, LY_SET_OPT_USEASLIST);
+            LY_CHECK_RET(ly_set_add(&lydctx->unres_node_type, *node, LY_SET_OPT_USEASLIST, NULL));
         }
         ret = LY_SUCCESS;
     }
@@ -430,7 +430,7 @@
     ret = lyd_create_meta(parent, meta, mod, name, name_len, value, value_len, dynamic, value_hints, format, prefix_data,
                           ctx_snode);
     if (ret == LY_EINCOMPLETE) {
-        ly_set_add(&lydctx->unres_meta_type, *meta, LY_SET_OPT_USEASLIST);
+        LY_CHECK_RET(ly_set_add(&lydctx->unres_meta_type, *meta, LY_SET_OPT_USEASLIST, NULL));
         ret = LY_SUCCESS;
     }
     return ret;