monitor : do not trigger error when inventory unavailable

Under some startup scenarios, fan-monitor can start before the
InventoryManager service has populated inventory. This leads to
false-positives when marking fans non-functional.

This fix detects D-Bus exceptions and defaults the fans to functional.
They subscribe to the interfaces-added signal which will populate the
fans with correct values as soon as Inventory Manager completes startup.

Signed-off-by: Mike Capps <mikepcapps@gmail.com>
Change-Id: I14bff20da14cba3f5ef4b79763867b5cecab0267
diff --git a/monitor/fan.hpp b/monitor/fan.hpp
index d4bcb2d..8d9f3b2 100644
--- a/monitor/fan.hpp
+++ b/monitor/fan.hpp
@@ -213,8 +213,10 @@
      *
      * @param[in] functional - If the Functional property should
      *                         be set to true or false.
+     *
+     * @return - True if an exception was encountered during update
      */
-    void updateInventory(bool functional);
+    bool updateInventory(bool functional);
 
     /**
      * @brief Called by _monitorTimer to start fan monitoring some