Avoid g_critical() warning triggered by D-Bus API
Calling the org.freedesktop.Tracker.Miner Start() method could trigger a g_critical() warning if the miner was already started:
Tracker-CRITICAL **: 10:32:39.592: tracker_miner_start: assertion 'miner->priv->started == FALSE' failed'
Callers of the D-Bus API have no reliable way to know if the miner is already running, so we should just remove the warning and allow tracker_miner_start() to be called even when the miner is running. The same is true of Stop().