MAINT-8087: Use SHGetKnownFolderPath(), not SHGetSpecialFolderPath().

SHGetSpecialFolderPath() is deprecated, and empirically it appears to be
failing when the user name contains non-ASCII characters. The relevant
Microsoft documentation pages recommend calling SHGetKnownFolderPath()
instead.

Also, the SHGetSpecialFolderPath() calls had no error checking or reporting,
which is why we can only say it "appears to be" failing. Make sure that if
SHGetKnownFolderPath() fails, at least we try to tell somebody about it.