if read only backup engine can't find meta dirs, return NotFound() instead of IOError()

Summary: Read only backup engine return NotFound() on missing meta dir (for e2e test)

Test Plan: backupable_db_test

Reviewers: andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D60273

utilities/backupable/backupable_db.cc

@@ -560,7 +560,7 @@ Status BackupEngineImpl::Initialize() {
   {
     auto s = backup_env_->GetChildren(GetBackupMetaDir(), &backup_meta_files);
     if (!s.ok()) {
-      return s;
+      return Status::NotFound("%s not found", GetBackupMetaDir());
     }
   }
   // create backups_ structure