Return `Result` from all `generate_*` methods #2063

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	5m 29s
derive (1.56.0)	16s
derive (stable)	9s
doc	25s
build (1.56.0, wasm32-unknown-unknown)	5m 25s
build (stable, thumbv7em-none-eabi)	19s
test (1.56.0)	5m 31s
build (stable, wasm32-unknown-unknown)	27s
test (stable)	14s
minimal-versions / minimal-versions	33s
	18m 48s

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