Improve SPI error messages when using embedded_hal

[andber1]

I removed the error conversions to an Error::Io. Is there a reason for this?
This way you get more useful error messages. For example

called `Result::unwrap()` on an `Err` value: Io(Os { code: 90, kind: Uncategorized, message: "Message too long" })

instead of

called `Result::unwrap()` on an `Err` value: Io(Custom { kind: Other, error: "SimpleHalSpiDevice write transaction error" })

[golemparts]

While I'm not sure what the reasoning was for the conversions, I agree that the original error messages provide more useful information. I'll merge this in, unless someone has a specific reason to keep the error mapping.