exception message for unrecognized source IDs in lookupDestinationIds() should have more detail

    if (!empty($source_id_values)) {
      throw new MigrateException("Extra unknown items in source IDs");
    }

This is not much detail to help a developer track down a problem. The message should:

- list the unknown source IDs. Those are just the array keys in $source_id_values at that point
- state which migration is involved. This is typically called for a migration lookup, so it could be any of a number of migrations that are being used for lookups in the migration actually being run