Customize "Critical Error The Selected Category Does Not Exist"

[TeenStorm]

I've been searching other posts and haven't come across something that caught my attention yet.  Search engines have organically indexed URL's to my website.  I have, however, imported a totally different database from an offline development platform to my live website such that certain tables like expired category references no longer exist, hence, when you click on a search engine's indexed link that points to an invalid category you get the "Critical Error:  The Selected Category Does Not Exist" error page.  I want to customize this and/or insert a redirect to the main index.  I see a reference from Nibbler that led me to look at include/init.inc.php but this doesn't seem like the right place; can you redirect me where to look for this error and I will see what I can do with it at my own expense?   

[Nibbler]

Find the error message in the language file and then search the code for the key associated with it.