Return `Result` from all `generate_*` methods #2098

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	6m 9s
test (1.65.0)	3m 34s
build (1.65.0, wasm32-unknown-unknown)	6m 29s
test (stable)	14s
build (stable, thumbv7em-none-eabi)	16s
build (stable, wasm32-unknown-unknown)	20s
	17m 2s

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