OK, the 404 can be found by simply adding rubbish characters after your base Community url. Just anything so long as it doesn’t exist. Here you could use ….

https://communities.gainsight.com/hjshdj


For 403 you need to forbid access to get this error. The easiest way to do this is to create a custom page and give nobody (not even the admin) access to it. Then when you go to that custom page URL, you’ll see the 403. 

If you are logged in as the administrator, you will be able to edit the phrases from here.

For the 500 error, this is a little harder to force. But you can change/set the message using the phrases page. Go to your Community admin console and select it from Customisation and then Phrases. The url will be something like this….

https://yourcommunity.com/translation/overview

You will need to edit the module is “Forum” and the key is “error.500”
 

I think there is also 400 error page as well and that is really, really tricky one