You get multiple error messages and it appears the only work-around is to give users Local Administrator access prior to performing a normal Smart Upgrade. Power user access is not enough, it must be Local Administrator access. This can be done with a Group Policy Object in AD.
Please note that this information relates to attempting to upgrade a 8.5 multi-user client to 8.5.1 multi-user. If you have different clients your milage will vary.