Monitor: Support hwmon service offline during startup

It is possible for fan-monitor to startup before the Hwmonitor service,
causing unhandled exceptions that block system initialization. This fix
catches the exception until a proper hwmon presence detector is
deployed.

If the exception is caught, this code change forces a re-subscription
during the poweron event to ensure tach sensors will receive published
updates upon resumption of the hwmon service.

Signed-off-by: Mike Capps <mikepcapps@gmail.com>
Change-Id: I8e696e747c432d7a6f696c5ccd9dab73abf7708f
diff --git a/monitor/system.hpp b/monitor/system.hpp
index a690567..328c958 100644
--- a/monitor/system.hpp
+++ b/monitor/system.hpp
@@ -186,12 +186,11 @@
     json captureSensorData();
 
     /**
-     * @brief Builds a mapping for sensors to be identified
-     *        for a given service name.
+     * @brief creates a subscription (service->sensor) to take sensors
+     *        on/offline when D-Bus starts/stops updating values
      *
-     * @return a map of service_name->[sensor1,sensor2...]
      */
-    SensorMapType buildNameOwnerChangedMap() const;
+    void subscribeSensorsToServices();
 
     /**
      * @brief Retrieve the configured trust groups