CLI: MaintenanceTool install should succeed if packages already installed

When calling `MaintenanceTool install` with packages that are already installed, the installer prints "Components already installed." and returns a non-zero exit code (3).
When installing Qt e.g. using provisioning scripting, this cannot be told from an actual error in a robust way, as they also return with exit code 3.

I think `MaintenanceTool install` should return 0 when all packages are already installed, which would be the standard behavior from other package managers (apt, yum, brew, chocolatey, you name it).