Return `Result` from all `generate_*` methods #1584

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	3m 36s
minimal-versions	35s
build (1.60.0, wasm32-unknown-unknown)	5m 20s
test (1.60.0)	3m 42s
build (stable, thumbv7em-none-eabi)	13s
test (stable)	13s
build (stable, wasm32-unknown-unknown)	9s
	13m 48s

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