Report errno from libblkid code

[jlebon]

Right now, we're translating libblkid errors into generic codes like RESULT_OPEN_FAILED. But this doesn't capture why the operation failed. We should report the actual errno value too to make debugging easier.

Looking at the code, we're just translating that into an error string anyway, so it doesn't seem like we actually need to handle specific failure types differently. Because one cannot reliably access errno from Golang, one suggestion is to just do the error prefixing on the C side (this is what #846 did when it used the SELinux API directly before switching to setfiles: https://github.com/coreos/ignition/compare/d8772fbc6a61a4ddf9b47ff90feeaa12d0d38e34..af996492e367bebc0ddf0bc4e5f8d5cbf9749623#diff-e4e61a4b3c86fbfe43c406733947227dL77).