Return `Result` from all `generate_*` methods #2095

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	6m 51s
test (1.65.0)	3m 35s
build (1.65.0, wasm32-unknown-unknown)	5m 27s
test (stable)	14s
build (stable, thumbv7em-none-eabi)	9s
build (stable, wasm32-unknown-unknown)	13s
	16m 29s

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