Return `Result` from all `generate_*` methods #1653

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 36s
clippy	3m 28s
build (1.65.0, wasm32-unknown-unknown)	3m 32s
build (stable, thumbv7em-none-eabi)	29s
build (stable, wasm32-unknown-unknown)	16s
minimal-versions	30s
test (1.65.0)	3m 38s
test (stable)	15s
	15m 44s

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