Did you grab some backups before attempting the upgrade?
I'd suggest opening a support case since that message means something in the post upgrade clean up didn't happen. It might also mean something else in the upgrade also had issues. Check /var/log/upgrade/ for the upgrade log.
The message itself is a red herring, not finding root/svn/mercury isn't a problem since the new main code path should be root/git/mercury (the source code repository was changed from svn to git a few years ago). But the health of the server should definitely be checked out by a support engineer.
8.2.2 is an tested and approved intermediate upgrade step before going to 8.2.6, so if you wanted to by-pass calling support, you could make sure you have a complete backup image, reinstall 8.2.1.1, restore the backup, then upgrade to 8.2.2 before going to 8.2.6.1.