checkbox-touch: fix freeze when app cache dir is missing

This patch fixes a problem, where Checkbox Touch freezes when app's cache dir
is not present of a filesystem.

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1399107

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge of lp:~kissiel/checkbox/fix-1399107/ by tarmac [r=sylvain-pineau][bug=][author=kissiel]"