Return `Result` from all `generate_*` methods #2009

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	5m 18s
build (1.56.0, wasm32-unknown-unknown)	3m 45s
test (1.56.0)	3m 43s
build (stable, thumbv7em-none-eabi)	10s
test (stable)	13s
build (stable, wasm32-unknown-unknown)	11s
minimal-versions / minimal-versions	25s
	13m 45s

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