That error says the qa-install.php file is missing. So you need to put back the qa-install.php file. (Download Q2A again if you don't have the zip file any more and get it from that.)
I think in the docs it's recommended to remove it for security reasons, but it's required when running upgrades and installing plugins that use database tables. The security implications are very minor, so I would recommend keeping the file. At most, you could rename the file to something unique but keep it there, then rename it back to qa-install.php whenever you need to install it.