Windows error message has wrong encoding

[selvanair]

Originally posted by @mastezont in OpenVPN/openvpn-gui#491 (comment)

Windows 7, OpenVPN version 2.6.3, OpenVPN GUI 11.41.0.0, OS language and OpenVPN GUI Russian. In the "display status" window, part of the message in Russian with the wrong character set (highlighted in the screenshot). If you select "show log" and open it in Notepad++, it shows character set Cyrillic Macintosh instead of the usual Cyrillic Windows-1251, while the word "Элемент" is displayed incorrectly as "Ёлемент" (highlighted in the screenshot).
This error has appeared since version 2.6.0, if memory serves me correctly.

Originally posted by @mastezont in OpenVPN/openvpn-gui#491 (comment)

[selvanair]

In error.c in openvpn we have

/* format a windows error message */
    {
        char message[256];
        struct buffer out = alloc_buf_gc(256, gc);
        const int status =  FormatMessage(FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_FROM_SYSTEM
            | FORMAT_MESSAGE_ARGUMENT_ARRAY, NULL, errnum, 0,
            message, sizeof(message), NULL);

This will call the ANSI version and return "message" using the current code page while the implicit assumption is that all strings are unicode in utf-8 encoding. I think we should call FormatMessageW() here and convert the result to utf-8.