Return `Result` from all `generate_*` methods #2010

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	3m 35s
build (1.56.0, wasm32-unknown-unknown)	5m 22s
test (1.56.0)	3m 40s
build (stable, thumbv7em-none-eabi)	11s
build (stable, wasm32-unknown-unknown)	27s
test (stable)	11s
minimal-versions / minimal-versions	22s
	13m 48s

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