ldb to allow db with --try_load_options and without an options file

Summary:
This is to fix tools/check_format_compatible.sh. The tool try to open
old versions of rocksdb with the provided options file. When options
file is missing (e.g. rocksdb 2.2), it should still proceed with default
options.
Closes https://github.com/facebook/rocksdb/pull/3232

Differential Revision: D6503955

Pulled By: yiwu-arbug

fbshipit-source-id: e44cfcce7ddc7d12cf83466ed3f3fe7624aa78b8

tools/ldb_cmd.cc

@@ -315,7 +315,7 @@ void LDBCommand::OpenDB() {
   if (!create_if_missing_ && try_load_options_) {
     Status s = LoadLatestOptions(db_path_, Env::Default(), &options_,
                                  &column_families_, ignore_unknown_options_);
-    if (!s.ok()) {
+    if (!s.ok() && !s.IsNotFound()) {
       // Option file exists but load option file error.
       std::string msg = s.ToString();
       exec_state_ = LDBCommandExecuteResult::Failed(msg);