Return `Result` from all `generate_*` methods #1305

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	6m 17s
test (1.65.0)	5m 20s
build (1.65.0, wasm32-unknown-unknown)	3m 39s
test (stable)	12s
build (stable, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	21s
minimal-versions / minimal-versions	34s
	16m 46s

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