Better error messages when upgrading unclean store

Now produces more helpful error messages when people attempt to upgrade store files
that have not been shut down cleanly. We don't tell people that we cannot upgrade
from some weird binary string of a version number, but instead correctly tell them
that the store file has not been shut down properly. When this happens, they should
open the store again in the old version, shut it down properly, and then try
upgrading it again. That should do the trick.