Return `Result` from all `generate_*` methods #2364

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 57s
build (1.65.0, wasm32-unknown-unknown)	3m 57s
minimal-versions	1m 4s
build (stable, thumbv7em-none-eabi)	1m 9s
test (1.65.0)	3m 48s
build (stable, wasm32-unknown-unknown)	37s
test (stable)	44s
test (nightly)	47s
doc	20s
	16m 23s

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