lib.types: Add unique like uniq, but custom errors

Couldn't extend types.uniq and it had a silly name anyway.
Now we can have better error messages.

lib/options.nix

@@ -134,6 +134,12 @@ rec {
       throw "The unique option `${showOption loc}' is defined multiple times. Definition values:${showDefs defs}"
     else (head defs).value;
 
+  mergeUniqueOption = { message }: loc: defs:
+    if length defs == 1
+    then (head defs).value
+    else assert length defs > 1;
+      throw "The option `${showOption loc}' is defined multiple times.\n${message}\nDefinition values:${showDefs defs}";
+
   /* "Merge" option definitions by checking that they all have the same value. */
   mergeEqualOption = loc: defs:
     if defs == [] then abort "This case should never happen."