Summary:
Bug fix for status returned being overridden by Status::NotFound in
DBImpl::OpenForReadOnlyCheckExistence. This was casuing some service
owners to misinterpret the actual error and take appropriate steps.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7972

Reviewed By: riversand963

Differential Revision: D26499598

Pulled By: akankshamahajan15

fbshipit-source-id: 05e9fedbe2a2e0e53135760f8ff578a2816d2b8e