Return `Result` from all `generate_*` methods #1736

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 50s
build (1.65.0, wasm32-unknown-unknown)	5m 21s
minimal-versions	1m 1s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	17s
test (1.65.0)	6m 26s
test (stable)	23s
	17m 31s

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