persistent-nodemap: catch the right exception on python
On Python 2, Missing file are IOError, not OSError, apparently.
Differential Revision: https://phab.mercurial-scm.org/D9809
$ 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