Return `Result` from all `generate_*` methods (#1379) #1595

Job	Run time
minimal-versions	47s
build (1.60.0, thumbv7em-none-eabi)	3m 38s
build (1.60.0, wasm32-unknown-unknown)	5m 18s
test (1.60.0)	6m 29s
build (stable, thumbv7em-none-eabi)	20s
test (stable)	23s
build (stable, wasm32-unknown-unknown)	17s
	17m 12s

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