Werk #7167: Added installation verification for automatic agent updates on windows

Previously, a broken agent update on Windows (i.e. the msiexec command did return properly, but the installation is broken) did not get recognized. Hence the agent updater recognized this situation as a successful update and did not start any further attempt to install the new MSI package.

This is now mitigated by investigating the new installation after an update. If the update did not take place as expected, the agent update will exit with an appropriate error and retry on next agent updater run. This will also be visible within the Checkmk agent check and the update status on the Checkmk server.

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