chore: add fspy::error::SpawnError for detailed error reporting #27

Job	Run time
Test (ubuntu-latest, x86_64-unknown-linux-gnu)	1m 56s
Test (namespace-profile-mac-default, aarch64-apple-darwin)	40s
Test (windows-latest, x86_64-pc-windows-msvc)	3m 36s
Lint	1m 27s
done	2s
	7m 41s

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