[cli] do not crash when listing unaccessible directories

When the directory is not accessible, instead of crash like:

```
lua coroutine: runtime error: /opt/app-root/src/src/apicast/cli/filesystem.lua:50: attempt to call a nil value
```

Get a debug log like:
```
 /usr/local/share/lua/5.1/lfs.lua:513: cannot open /usr/local/openresty/nginx/fastcgi_temp : Permission denied
```

gateway/src/apicast/cli/filesystem.lua

@@ -13,14 +13,17 @@ local co_yield = coroutine.yield
 local co_create = coroutine.create
 local co_resume = coroutine.resume
 
+local noop = function () end
+
 --- Safely try to get directory iterator
 local function ldir(dir)
   local ok, iter, state = pcall(pl_path_dir, dir)
 
   if ok then
     return iter, state
   else
-    return nil, iter
+    ngx.log(ngx.DEBUG, 'error listing directory: ', dir, ' err: ', iter)
+    return noop
   end
 end