statfs: make getfstype() raise OSError
It's better for getfstype() function to not suppress an error. Callers can
handle it as necessary. Now "hg debugfsinfo" will report OSError.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state