Your best bet is to use a straight HTML page and not a server page like aspx. IIS can serve this up without needing the .net framework and it won't need to be precompiled either.
Now to your problem, I believe the problem is Kentico is trying to process the page
/SitePages/Errors/error500.aspx as a portal page. So you need to do a few things:
- place that file in a directory in which you can target with the second bullet point
- tell Kentico to not process that directory.
Best bet is to place your
error500.aspx page in say this directory
/sitecodename/noprocess/. Then in Kentico Settings>URLs and SEO>URL Format, add the path to that directory in the Excluded URLs textbox. This will tell Kentico to not process the pages in that directory.