Return `Result` from all `generate_*` methods (#1379) #1743

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	6m 21s
minimal-versions	1m 2s
build (1.65.0, wasm32-unknown-unknown)	5m 16s
test (1.65.0)	3m 32s
build (stable, thumbv7em-none-eabi)	17s
test (stable)	32s
build (stable, wasm32-unknown-unknown)	22s
	17m 22s

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