windows/fs: handle _open_osfhandle() failure correctly
windows/fs: handle _open_osfhandle() failure correctly

Until now we assumed that _open_osfhandle() would set _doserrno on
failure. This assumption was very wrong in one obvious case, namely when
the CRT file descriptor table would fill up. In that case errno is set
to EMFILE, but GetLastError() returns zero - which makes sense because
it's not a win32 error but rather a CRT problem.

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