fix(routing): return correct error

Author: tkan145

gateway/src/apicast/policy/routing/rule.lua

@@ -74,8 +74,8 @@ function _M.new_from_config_rule(config_rule)
     self.owner_id = tonumber(config_rule.owner_id)
     return self
   else
-    return nil, 'failed to initialize upstream from url: ',
-                config_rule.url, ' err: ', err
+    return nil, 'failed to initialize upstream from url: ' ..
+                config_rule.url .. ' err: ' .. (err or 'unknown')
   end
 end