[dbus-passive] Add threshold fan failure

When a threshold is crossed for a monitored sensor,
assert fan failure.

Tested-by: Changed a sensor threshold so that its current
reading made the threshold asserted and noticed via print
messages that the sensor went into failure state. Also
noticed fans ramp. Wrote unit test to verify sensor can
move in and out of error state correctly.

Change-Id: I83182536e4874eaba97f3f1d48d53ac110fba833
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/interfaces.hpp b/interfaces.hpp
index bf45545..89ef128 100644
--- a/interfaces.hpp
+++ b/interfaces.hpp
@@ -29,6 +29,11 @@
     }
 
     virtual ReadReturn read(void) = 0;
+
+    virtual bool getFailed(void) const
+    {
+        return false;
+    }
 };
 
 /*