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

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/options.nix b/lib/options.nix
index a16a980398d..d2eae3ae3a9 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -101,7 +101,7 @@ rec {
   mergeOneOption = loc: defs:
     if defs == [] then abort "This case should never happen."
     else if length defs != 1 then
-      throw "The unique option `${showOption loc}' is defined multiple times, in ${showFiles (getFiles defs)}."
+      throw "The unique option `${showOption loc}' is defined multiple times, in:\n - ${concatStringsSep "\n - " (getFiles defs)}."
     else (head defs).value;
 
   /* "Merge" option definitions by checking that they all have the same value. */