workspace: streamline error types for load() functions #975

Job	Run time
build (nightly) / rust	17s
build (stable) / rust	17s
build (msrv) / rust	20s
build no_std (nightly)	0s
style (nightly)	0s
tests with miri (nightly)	0s
build no_std (stable) [w/o builder] / rust	10s
build no_std (stable) [Windows] / rust	56s
build no_std (stable) / rust	14s
style (stable) / rust	27s
style (msrv) / rust	22s
build no_std (msrv) / rust	20s
	3m 23s

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