module system: prettify a bit error when unique option defined twice

The error can be reproduced like:
```
$ nix-instantiate ./nixos -A system --arg configuration '
  { fileSystems."/".device = "nodev";
    boot.loader.grub.devices = [ "nodev" ];
    containers.t.config.imports = [ <nixpkgs/nixos/modules/virtualisation/amazon-image.nix> ];
  }'
```

Previously error was:
```
error: The unique option `containers.t.networking.hostName' is defined multiple times, in `/nix/var/nix/profiles/per-user/root/channels/nixpkgs/nixos/modules/virtualisation/amazon-image.nix' and `module at /home/danbst/dev/nixpkgs/nixos/modules/virtualisation/containers.nix:470'.
(use '--show-trace' to show detailed location information)
```

Now it is:
```
error: The unique option `containers.t.networking.hostName' is defined multiple times, in:
 - /nix/var/nix/profiles/per-user/root/channels/nixpkgs/nixos/modules/virtualisation/amazon-image.nix
 - module at /home/danbst/dev/nixpkgs/nixos/modules/virtualisation/containers.nix:470.
(use '--show-trace' to show detailed location information)
```

Related: https://github.com/NixOS/nixpkgs/issues/15747