`Error` on targets where `RawOsError` is not equal to `i32`

[newpavlov]

Previously, all Rust targets with std were using i32 for "raw" OS error codes, i.e. io::Error::raw_os_error/from_raw_os_error have accepted i32. We have encoded this assumption in Error::raw_os_error, fmt::Debug/Display, and From<Error> for io::Error implementations.

But in the recently added UEFI std implementation usize is used for error codes instead as per UEFI spec. It's reflected by the new (Nightly-only) type alias RawOsError

For now, we just added cfg-based exceptions for UEFI targets in the relevant internal implementations (see #566). But it may be worth to reconsider our use of NonZeroU32 for Error internal representation and return type of Error::raw_os_error.

cc @usamoi