Return `Result` from all `generate_*` methods #1588

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	3m 41s
minimal-versions	32s
build (1.60.0, wasm32-unknown-unknown)	5m 57s
build (stable, thumbv7em-none-eabi)	17s
test (1.60.0)	3m 43s
build (stable, wasm32-unknown-unknown)	13s
test (stable)	13s
	14m 36s

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