[BUG] Restoring volume stuck forever if the backup is already deleted. #1867

Describe the bug
Restoring volume stuck with null state forever if the backup is already deleted.

To Reproduce
Steps to reproduce the behavior:

1. Create a volume and write 100 mb data to it.
2. Take a backup.
3. Restore the backup and immediately delete the backup.
4. The restoring volume stuck and never recovers or become faulted.

Expected behavior
The restoring volume should become faulted after retrying for some time, there should be a time out for retry.

Log
longhorn-support-bundle_e636dffc-08d6-4fd1-8aad-7a5fe167b4d2_2020-10-08T22-41-51Z.zip

Backup volume - pvc-f8473372-108d-4667-a54b-23b99595de66
Backup name - backup-708a7f01339f467a
Restoring volume - restore-2
Time - ~2020-10-08 22:24:00

Environment:

• Longhorn version: Longhorn Master - 10/08/2020
• Kubernetes version: 19.2
• Node OS type and version: Ubuntu 18.04

Additional context
The backupfrom of the restoring volume is referring to a non-existent backup because the backup is deleted.