Summary:
- Completed the switch statement for all possible `Code` values (the only one missing was `kCompactionTooLarge`).
- Removed the default case so compiler can alert us if a new value is added to `Code` without handling it in `Status::ToString()`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7872

Test Plan:
verified the log message for this scenario looks right

```
2021/01/15-17:26:34.564450 7fa6845fe700 [ERROR] [/db_impl/db_impl_compaction_flush.cc:2621] Waiting after background compaction error: Compaction too large: , Accumulated background error counts: 1
```

Reviewed By: ramvadiv

Differential Revision: D25934539

Pulled By: ajkr

fbshipit-source-id: 2e0b3c0d993e356a4987276d6f8a163f0ee8be7a