clarify error msg for destructuring assignment #8097

Job	Run time
smoke-test-and-release-macos-x86	21m 14s
fmt	1m 17s
test-and-release-macos (record-history)	4m 56s
smoke-test-and-release-windows	8m 59s
test-and-release-macos	30m 35s
test-and-release-macos (cvc5)	5m 8s
test-and-release-macos (singular)	4m 23s
test-and-release-macos (no-std)	5m 10s
test-and-release-macos (no-alloc)	4m 21s
docs	5m 0s
smoke-test-and-release-linux	7m 26s
release	20s
	1h 38m 49s

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