Core: update error message for mismatched "event" placements #20383

Job	Run time
Test hosting with 3.12 on ubuntu-latest	3m 12s
Test Python 3.8 ubuntu-latest	4m 41s
Test Python 3.9 ubuntu-latest	4m 16s
Test Python 3.10 ubuntu-latest	4m 10s
Test Python 3.11 ubuntu-latest	4m 38s
Test Python 3.12 ubuntu-latest	6m 0s
Test Python 3.8 windows-latest	6m 44s
Test Python 3.12 windows-latest	8m 14s
Test Python 3.12 macos-latest	3m 21s
	45m 16s

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