filesystem_error exception uses wrong encoding for what()

[polkovnikov.ph@…]

If one tries to access locked file on Windows with RU_ru locale, he'll get an exception with "отказано в доступе" (access denied) returned by its what() method. It's written in 1251 encoding, and it's very inconvenient to work with. I think, that there should be at least wwhat() method to get wstring of exception in Unicode.