Good question. Currently the only error reason that could be returned here is "Ayo, wtf did you do with my upgrade file?.  That would most likely mean user shenanigans, so maybe the best thing to do would be to spell out in the error message (printed in line 798) something along the lines of "we expected to see upgrade.txt in the data directory, but it was removed. You may need to restart the node to retry the upgrade"