Adds a backupNotFound sentinel error that sdk
consumers can use to easily identify when a backup did not produce any backup data.
Also adds extra testing and mocks to support
testing inside the repo package.
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#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🌻 Feature
- [x] 🤖 Supportability/Tests
#### Test Plan
- [x] 💪 Manual
- [x] ⚡ Unit test
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