chore: add fspy::error::SpawnError for detailed error reporting (#8) #29

Job	Run time
Test (namespace-profile-mac-default, aarch64-apple-darwin)	47s
Lint	1m 46s
Test (ubuntu-latest, x86_64-unknown-linux-gnu)	2m 17s
Test (windows-latest, x86_64-pc-windows-msvc)	5m 16s
done	2s
	10m 8s

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