Core: improve error message for missing "game" entry in yaml (#4185) #63

Job	Run time
Test hosting with 3.12 on ubuntu-latest	3m 15s
Test Python 3.8 ubuntu-latest	4m 41s
Test Python 3.9 ubuntu-latest	4m 28s
Test Python 3.10 ubuntu-latest	4m 16s
Test Python 3.11 ubuntu-latest	4m 32s
Test Python 3.12 ubuntu-latest	5m 58s
Test Python 3.8 windows-latest	6m 7s
Test Python 3.12 windows-latest	8m 8s
Test Python 3.12 macos-latest	3m 19s
	44m 44s

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