Improve error msg for SstFileWriter Merge (#6261)

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

table/sst_file_writer.cc

@@ -70,7 +70,8 @@ struct SstFileWriter::Rep {
       if (internal_comparator.user_comparator()->Compare(
               user_key, file_info.largest_key) <= 0) {
         // Make sure that keys are added in order
-        return Status::InvalidArgument("Keys must be added in order");
+        return Status::InvalidArgument(
+            "Keys must be added in strict ascending order.");
       }
     }