fix issue 11232 - rewritten std.windows.syserror.sysErrorString

Fixed for Unicode-correctness; now works on non-English Windows systems.
Made nothrow.
Added unit test.

As it is a rewrite, it's probably easier to review by looking at the [new source](https://github.com/JakobOvrum/phobos/blob/syserrorstring/std/windows/syserror.d).