improve is_unique data check message (#5125)

tests/checks/is_unique.jinja

@@ -18,7 +18,7 @@
   )
   SELECT IF(
    (SELECT COUNT(*) FROM non_unique) > 0,
-   ERROR("Duplicates detected (Expected combined set of values for columns {{ columns }} to be unique.)"),
+   ERROR("Duplicates detected (Expected combined set of values for columns {{ columns }} to be unique{% if where %} where {{ where }}{% endif %}.)"),
    NULL
   );
 {% endmacro %}