When I upgraded my site using rb-site, I noticed that it created the errordocs dir, but did not install 500.html.  In my case, I wanted to use the one included with reviewboard.  I also noticed that errordocs was not packaged.

I'm not sure what the desired behavior is, but I wrote this patch assuming that leaving errordocs out of the package was just an oversight.  In my copy, I added it to the package, and made this change to rb-site to just link the site directory's errordocs dir to it.

I've done a test install on a development VM.  The only change I made from the standard install process was copying the errordocs directory into the installed package for the purposes of testing my change.