[hotfix][ci] Make NoticeFileChecker log severe issues at error level.

[xintongsong]

This PR upgrades logging level of severe issues in NoticeFileChecker from WARN to ERROR

Currently, it's a bit confusing which issue is causing the check failure when there are multiple warning logs.
https://dev.azure.com/apache-flink/apache-flink/_build/results?buildId=18950&view=logs&j=52b61abe-a3cc-5bde-cc35-1bbe89bb7df5&t=54421a62-0c80-5aad-3319-094ff69180bb&l=23626

[xintongsong]

cc @rmetzger

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[rmetzger]

Thanks a lot for fixing this inconsistency