Add error status for no_slowdown & low priority write (#6396)

Summary:
When `no_slowdown` is enabled, it returns `Status::Incomplete("Write stall")` if a stall would occur. This patch adds descriptive text for when `no_slowdown` and `low_pri` are enabled.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6396

Differential Revision: D19808978

Pulled By: cheng-chang

fbshipit-source-id: a53b0d25ed414c821a086531e0222027f925e627

db/db_impl/db_impl_write.cc

@@ -1458,7 +1458,7 @@ Status DBImpl::ThrottleLowPriWritesIfNeeded(const WriteOptions& write_options,
       return Status::OK();
     }
     if (write_options.no_slowdown) {
-      return Status::Incomplete();
+      return Status::Incomplete("Low priority write stall");
     } else {
       assert(my_batch != nullptr);
       // Rate limit those writes. The reason that we don't completely wait