Improve Error Message When wal_dir doesn't exist (#4874)

Summary:
Right now the error mesage when options.wal_dir doesn't exist is not helpful to users. Be more specific
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4874

Differential Revision: D13642425

Pulled By: siying

fbshipit-source-id: 9a3172ed0f799af233b0f3b2e5e35bc7ce04c7b5

db/db_impl_open.cc

@@ -421,7 +421,10 @@ Status DBImpl::Recover(
     // produced by an older version of rocksdb.
     std::vector<std::string> filenames;
     s = env_->GetChildren(immutable_db_options_.wal_dir, &filenames);
-    if (!s.ok()) {
+    if (s.IsNotFound()) {
+      return Status::InvalidArgument("wal_dir not found",
+                                     immutable_db_options_.wal_dir);
+    } else if (!s.ok()) {
       return s;
     }