Return `Result` from all `generate_*` methods #2363

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 56s
doc	15s
minimal-versions	22s
test (1.65.0)	5m 33s
build (1.65.0, wasm32-unknown-unknown)	5m 44s
test (stable)	51s
build (stable, thumbv7em-none-eabi)	39s
test (nightly)	39s
build (stable, wasm32-unknown-unknown)	28s
	18m 27s

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