xz: Add a function to print Windows specific error messages.

xz: Add a function to print Windows specific error messages.

Native Windows C API functions do not use errno, but instead have to
call GetLastError(). There is not an easy way to convert this error
code into a helpful message, so this creates a wrapper around the
slightly complicated FormatMessage() function.

The new message_windows_error() function calls message_error() under the
hood, so it will set the exit status to 1.