Make the online installer more robust against silly user errors

Background
I just tried upgrading Qt Creator 3.1.1 to 3.1.2. Unfortunately, I forgot to close Creator before performing the upgrade.

As a result (I think), I got the following message:

Error during installation process (qt.tools.qtcreator):
Error while extracting 'installer://qt.tools.qtcreator/3.1.2-0qt-creator-opensource-vs2010-32.7z': internal code: E_FAIL

I clicked "Retry" and got the same message. So, I clicked "Ignore" and the process finished.

Issue
The Maintenance Tool now thinks that I have Qt Creator 3.1.2 installed. However, Qt/Tools/QtCreator/bin/ contained no binaries.

Furthermore, since Qt Creator is a compulsory component, the Maintenance Tool would not let me uninstall it.

I had no way to forcibly reinstall Qt Creator. My only option was to uninstall and reinstall the entire SDK (including the libraries and MinGW).

Suggestion
Protect against mistakes like mine, which are silly but easily done.

Perhaps check that Qt Creator is not running (many other installers do this), or roll back if the process fails.