add string separation while composing error message (#7919)

Summary:
This will fix a missing string separation between `msg[n]` and `state_`.
Example of an error message how its looking now:
```
IO error: No space left on deviceWhile appending to file: /home/willi/src/stable-3.7/tmp/arangosh_CL6EFQ/shell_client/single1/data/engine-rocksdb/126426.sst: No space left on device
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7919

Reviewed By: ajkr

Differential Revision: D26242246

Pulled By: jay-zhuang

fbshipit-source-id: 5d9a0997a410aecfb3781478e57395d3d937bb84

util/status.cc

@@ -150,6 +150,9 @@ std::string Status::ToString() const {
   }
 
   if (state_ != nullptr) {
+    if (subcode_ != kNone) {
+      result.append(": ");
+    }
     result.append(state_);
   }
   return result;