message: Add duplicated benchmark ID to assertion message.
diff: @@ -290,7 +290,8 @@ impl<'a, M: Measurement> BenchmarkGroup<'a, M> {
  assert!(
  !self.all_ids.contains(&id),
  - "Benchmark IDs must be unique within a group."
  + "Benchmark IDs must be unique within a group. Encountered duplicated benchmark ID {}",
  + &id
  );
  id.ensure_directory_name_unique(&self.criterion.all_directories);
  