Return `Result` from all `generate_*` methods #1585

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	6m 34s
minimal-versions	36s
build (1.60.0, wasm32-unknown-unknown)	6m 26s
build (stable, thumbv7em-none-eabi)	19s
test (1.60.0)	3m 48s
build (stable, wasm32-unknown-unknown)	8s
test (stable)	26s
	18m 17s

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