shelve: fix a missing variable in the exception handler for delete
Caught by pytype. I haven't paid much attention to the progress of this
extension, but I *think* this was the intent.
Differential Revision: https://phab.mercurial-scm.org/D7457
$ 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