Rather than overwriting /etc/apt/sources.list in base.postinst script with nearly static, predefined values that does not reflect installed distribution set of repos (contrib, non-free) and installation environment, update apt-setup generators to avoid non-preseedable error messages when mirrors isn't available.
This could happen for network installs in isolated environment, where access to publicly hosted security and updates mirrors isn't available.
While there, fix non-preseedable error message display when target system has no network interface.
Rather than overwriting /etc/apt/sources.list in base.postinst script with nearly static, predefined values that does not reflect installed distribution set of repos (contrib, non-free) and installation environment, update apt-setup generators to avoid non-preseedable error messages when mirrors isn't available.
This could happen for network installs in isolated environment, where access to publicly hosted security and updates mirrors isn't available.
While there, fix non-preseedable error message display when target system has no network interface.