#307 Misleading "successfully upgraded" message after failed upgrade

[David Bolton]

After a failed upgrade, the final message from the installer is that the upgrade was successful (see screenshot).

Steps that led me to this scenario:

1. Open old version of VeraCrypt
2. Help > User's Guide
3. Close VeraCrypt (but accidentally leave User's Guide open)
4. Install new version of VeraCrypt
5. Cancel install when it complains about not being able to write to the docs folder
6. Wait for misleading message that reads: "VeraCrypt has been successfully upgraded. However, before you can start using it, the computer must be restarted."