console: raise error if lua_encode failed

lua_encode can raise an exception (e.g. from luaL_checkfield) or return
`nil, err` result. Handle it in lbox_console_format_lua, similar to
lbox_console_format_yaml.

When fixed, it will expose another issue - nil value can not be
serialized:

```
tarantool> \set output lua
true;
tarantool> 1, nil, 2
1, {error = "console: exception while formatting the output:
    \"serializer: unexpected data (nd.field.size 0 nd.field.type 5)\""}, 2;
tarantool>
```

Fix this too.

Part of tarantool#6781
Part of tarantool#6934

NO_DOC=bugfix
NO_TEST=not a visible change
NO_CHANGELOG=not a visible change

src/box/lua/console.c

@@ -118,7 +118,17 @@ lbox_console_format_lua(struct lua_State *L)
 
 	lua_replace(L, 1);
 	lua_settop(L, 1);
-	return lua_encode(L, serializer_lua, &opts);
+	int ret = lua_encode(L, serializer_lua, &opts);
+	if (ret == 2) {
+		/*
+		 * Nil and error object are pushed onto the stack.
+		 */
+		assert(lua_isnil(L, -2));
+		assert(lua_isstring(L, -1));
+		return luaL_error(L, lua_tostring(L, -1));
+	}
+	assert(ret == 1);
+	return ret;
 }
 
 /*

src/box/lua/serialize_lua.c

@@ -955,7 +955,7 @@ dump_root(struct lua_dumper *d)
 
 	luaL_checkfield(d->L, d->cfg, lua_gettop(d->L), &nd.field);
 
-	if (nd.field.type != MP_ARRAY || nd.field.size != 1) {
+	if (nd.field.type != MP_ARRAY || nd.field.size > 1) {
 		d->err = EINVAL;
 		snprintf(d->err_msg, sizeof(d->err_msg),
 			 "serializer: unexpected data "