Improve error raised when returning None from `elaborate` #3642

Job	Run time
document	51s
check-links	49s
test (3.9)	2m 44s
smoke (amaranth-lang/amaranth-boards)	39s
test (3.10)	2m 39s
smoke (amaranth-lang/amaranth-stdio)	54s
test (3.11)	2m 41s
smoke (amaranth-lang/amaranth-soc)	52s
test (3.12)	4m 42s
test (3.13)	4m 22s
test (pypy-3.9)	4m 41s
test (pypy-3.10)	4m 45s
test (3.14-dev)	1m 55s
publish-docs	7s
publish-docs-dev	0s
required	1s
publish-schemas	5s
publish-package	9s
publish-release	0s
	32m 56s

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