Raise specific errors after upstream retries exhausted

[iHiD]

Closes #8588

Summary

• Added SnippetGeneratorTemporarilyUnavailable error to Iteration::GenerateSnippet and LocCounterTemporarilyUnavailable error to Iteration::CountLinesOfCode
• After exhausting 3 retries on 502/503/504 responses, these named errors are raised instead of the raw RestClient exceptions
• This gives each failure a distinct identity in Sentry so they can be manually ignored unless they escalate

Test plan

• Updated existing "raises after max retries exhausted" tests to expect the new error classes
• All 20 tests pass for both commands

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