Clarify when a verify UNAVAILABLE gRPC error is expected

[yvetteyuanw]

Clarify that an UNAVAILABLE gRPC error is expected when the device is still performing install operation on the primary control card, even if the device is reachable.

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[coveralls]

Pull Request Test Coverage Report for Build 11717428714

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 1.142%

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Totals
Change from base Build 11714172457:	0.0%
Covered Lines:	166
Relevant Lines:	14542

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💛 - Coveralls