Make error message for missing "openapi" key clearer. #166

Job	Run time
Compatibility test	4m 59s
Example packages / Linux (5.9)	18m 6s
Example packages / Windows (6.0)	2s
Example packages / Linux (5.10)	15m 24s
Example packages / Windows (nightly-6.0)	3s
Example packages / Linux (6.0)	17m 48s
Example packages / Windows (nightly-main)	2s
Example packages / Linux (nightly-6.0)	22m 11s
Integration test / Windows (6.0)	1s
Integration test / Linux (5.9)	1m 53s
Example packages / Linux (nightly-main)	29s
Integration test / Linux (5.10)	1m 37s
Integration test / Windows (nightly-6.0)	3s
Soundness / API breakage check	0s
Integration test / Linux (6.0)	1m 58s
Integration test / Windows (nightly-main)	1s
Integration test / Linux (nightly-6.0)	2m 15s
Integration test / Linux (nightly-main)	30s
Soundness / Broken symlinks check	5s
Soundness / Documentation check	1m 37s
Soundness / Format check	34s
Soundness / License headers check	6s
Soundness / Python lint check	6s
Soundness / Shell check	28s
Soundness / Unacceptable language check	2s
Soundness / YAML lint check	0s
Unit tests / Unit tests / Linux (5.9)	1m 36s
Unit tests / Unit tests / Windows (6.0)	3s
Unit tests / Unit tests / Linux (5.10)	1m 37s
Unit tests / Unit tests / Windows (nightly-6.0)	1s
Unit tests / Unit tests / Linux (6.0)	1m 52s
Unit tests / Unit tests / Windows (nightly-main)	3s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 7s
Unit tests / Unit tests / Linux (nightly-main)	34s
	1h 38m 13s

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