Improve error message for custom type with fields but no constructor #12676

Job	Run time
rustfmt	18s
lint-build	58s
validate	19s
test-wasm	1m 25s
test (stable, aarch64-unknown-linux-gnu)	41s
test (stable, x86_64-apple-darwin)	9m 5s
test (stable, x86_64-unknown-linux-musl)	42s
test (stable, aarch64-apple-darwin, macos-latest, arm64, cargo, true)	4m 53s
test (stable, x86_64-pc-windows-msvc)	5m 2s
test (stable, x86_64-unknown-linux-gnu)	3m 48s
test (stable, aarch64-unknown-linux-musl)	50s
test-projects	2m 25s
	30m 26s

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