Improve error messages for empty Mux1H and PriorityMux (backport #4391) #6783

Job	Run time
Integration Tests	7m 6s
Formatting	3m 15s
ci (ubuntu-20.04, 8, 2.13.10, 2.4, firtool-1.43.0)	12m 40s
ci (ubuntu-20.04, 8, 2.12.17, 2.4, firtool-1.43.0)	13m 1s
compile project with mill	9m 43s
Test Chisel Scala-CLI Example	3m 29s
Standard Library Tests (2.13.10)	2m 13s
Standard Library Tests (2.12.17)	2m 21s
check tests	1s
all tests passed	4s
publish	0s
	53m 53s

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