Adhere to QT_ALLOW_DOWNLOAD in mocwrapper

As a Qt Framework user
I want that if I switch QT_ALLOW_DOWNLOAD to off the Qt License Service does not try to make a network request to obtain a license,
So that I can control if there are outbound connections from my machine.

If QT_ALLOW_DOWNLOAD is disabled and there is no local license file or previous reservation the mocwrapper should exit with error thus stopping the compilation.