Windows io::Error: also format NTSTATUS error codes
`NTSTATUS` errors may be encoded as `HRESULT`, see [[MS-ERREF]](https://msdn.microsoft.com/en-us/library/cc231198.aspx). These error codes can still be formatted using `FormatMessageW` but require some different parameters to be passed in.
I wasn't sure if this needed a test and if so, how to test it. Presumably we wouldn't want to make our tests dependent on localization-dependent strings returned from `FormatMessageW`.
Users that get an `err: NTSTATUS` will need to do `io::Error::from_raw_os_error(err|0x1000_0000)` (the equivalent of [`HRESULT_FROM_NT`](https://msdn.microsoft.com/en-us/library/ms693780(VS.85).aspx))
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