launcher-tree: Skip non-exist dirs when testing the cache
launcher-tree: Skip non-exist dirs when testing the cache

We are watching "~/.local/share/applications" even when it doesn't
exist, but in that case we shall not make a fuzz about it and test
the cache against it either.

Fixes: http://bugs.meego.com/show_bug.cgi?id=1647

meego-netbook-ux/meego-panel-applications,meego-netbook-ux/meego-panel-applications