A handful of the error sentinel values in graph/errors.go were only around to comply with an old expectation that IsErr funcs have a matching sentinel. That assumption is out of date, and there's no point in keeping around unused sentinels.
This PR makes three changes to clean up that issue:
1. remove unused sentinels (unused is qualifed as "not produced").
2. fix external pacakges which referenced those errors in tests alone by replacing those sentinels with odata errors.
3. centralize odata error test code to help support upstream package odata error creation.
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#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🧹 Tech Debt/Cleanup
#### Test Plan
- [x] ⚡ Unit test
- [x] 💚 E2E
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