Summary:
Reword the error message when keys are not added in strict ascending order.
Specifically, original error message is not clear when application tries to
call SstFileWriter::Merge() with duplicate keys.
Test plan (dev server)
```
make check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6261
Differential Revision: D19290398
Pulled By: riversand963
fbshipit-source-id: 4dc30a701414e6894db2eb024e3734470c22b371
946c43a026