Check the basic installation preconditions before the installation starts

If the user tries to install Qt to an OS version, which is not supported, the error message is not very easily understandable. The error message is also given after the downloads, when the installation is started.

• We should tell the user exactly what the problem is. The --verbose switch actually tells the OS version is too old
• We should do the basic pre-conditions check, like the OS version before the installation. It would be nice to also check, if there is the toolchain, required by the Qt modules to be installed.