app: do not free non-allocated DBus manager.
While this issue was unseen so far on common desktop machines, the CI build encountered it, hence failing 6 of the unit tests. A connection to the bus could not be established hence the dbus_manager was never allocated, and finally it would crash at exit if we tried to unref it unconditionnally. Use g_clear_object() instead. Also add some stderr output for easier debugging, for when one of the 2 possible error cases might happen (as documented by g_bus_own_name()).
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