Is there an option that we provide more meaningful message to the users that obtained non-full archive package via download. Currently the error message says only "Non 7z archive".

It would be much more user friendly to hint that he/she might have got error during downloading.
For example "if you obtained the package from Internet, please try to download it again". See for example http://nvidia.custhelp.com/app/answers/detail/a_id/2649/~/after-downloading-drivers-upon-install-i-get-a-non-7-zip-archive-error.