[services] Backup - Fix premature errors crash

Summary:
Depends on D3532

In CreateNewBackupReactor when an error occurs before the putReactor is created, there is a crash, because we try to use the putReactor in doneCallback but it's nullptr.

Test Plan:

cd services
yarn run-backup-service

Reviewers: geekbrother, varun, palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3533