feat: add message about mirror when nvd is down #8538

Job	Run time
Documentation	1m 3s
Linux tests (3.9)	4m 18s
Linux tests (3.10)	4m 0s
Linux tests (3.11)	2m 9s
Long tests on Python 3.13 (synchronous)	4m 15s
Windows long tests	54m 47s
Long tests on Python 3.13 (language parsers)	4m 44s
Linux tests (3.12)	1m 16s
Long tests on Python 3.13	9m 24s
Tests that may fail due to network or HTML	3m 28s
Long tests on Python 3.13 (scanners)	30m 16s
	1h 59m 40s

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