broadcast: report invalid settings #9252

Job	Run time
1/4	12m 16s
2/4	14m 25s
4/4	14m 33s
macos 2/5	20m 18s
py-3-latest	18m 16s
3/4	10m 11s
_remote_background_indep_poll	4m 52s
_remote_background_indep_tcp	6m 40s
macos 1/5	25m 17s
flaky	7m 11s
codecov	9s
	2h 14m 8s

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