Return `Result` from all `generate_*` methods #2366

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 56s
doc	20s
build (1.65.0, wasm32-unknown-unknown)	6m 11s
minimal-versions	44s
build (stable, thumbv7em-none-eabi)	32s
test (1.65.0)	6m 16s
build (stable, wasm32-unknown-unknown)	38s
test (stable)	19s
test (nightly)	38s
	19m 34s

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