Error message for upgrade failure due to expired license is wrong

When admins upgrading Confluence encounter this issue:
https://confluence.atlassian.com/display/CONFKB/Upgrade+fails+due+to+expired+license

The error message tells them to restore their database and home directory to the pre-upgrade state. In reality, no upgrade tasks are run when this happens, so the data remains untouched and they should be able to downgrade back to the old version without restoring a backup.

We should change the error message to indicate that that is the case.