Lesson: Don't commit when you're tired.

If DBus is not available, just say we're deactivating the plug-in instead of
showing that scary critical.

This fixes a critical when tumbler isn't installed.

https://bugzilla.gnome.org/show_bug.cgi?id=679262

https://bugzilla.gnome.org/show_bug.cgi?id=676136

Conflicts:
	src/librygel-renderer/rygel-av-transport.vala

127244d6 introduced a mtime of int64.MAX as a hack to mark items as a
place-holder in the db, unfortunately not all paths handling with mtime were
updated to reflect this, so uploaded items were harvested (due to size
difference) but under some circumstances the mtime wasn't updated properly.

Signed-off-by: Gheyret Kenji <gheyret@gmail.com>