background: Be robust against missing directories, and warn otherwise

We want to track the busy state of the source. To do that we will keep
a count of the number of pending operations. Not having a check for
NOT_FOUND makes it slightly easier to do that. Not a big deal, but I
think it outweighs the downfall of showing an extra warning.

Plus we are trying to create the directories before using them, so the
warning means something fishy has been going on with your system.

https://bugzilla.gnome.org/show_bug.cgi?id=708943