Return `Result` from all `generate_*` methods #1651

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 16s
build (1.65.0, wasm32-unknown-unknown)	3m 31s
build (stable, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	19s
clippy	3m 46s
minimal-versions	27s
test (1.65.0)	3m 41s
test (stable)	33s
	17m 47s

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