I am not very familar with Confluence, but if altassian programming style holds true, look for a file named
*404*.*. It might be a jsp file or a vm file. You might be able to tell by looking at the url when a page not found error occurs. There might also be a file with the word "*error* in its base file name. Try adding your text and link to either file. I would suspect it should be the 404 file though. You might need to restart Confluence and flush your browser cache.
Please be aware whenever Confluence is upgraded, these changes will need to be reapplied.
If you want to modify existing text, you might be able to do it through updating your language pack.
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