conman:  QCSAPI failures should not be silent.

Change-Id: I38c2cdf748c9fc2251c8c0c846a4de7b593f9f91
diff --git a/conman/interface.py b/conman/interface.py
index 1b77b18..e5a89a1 100755
--- a/conman/interface.py
+++ b/conman/interface.py
@@ -479,7 +479,11 @@
     self._events = []
 
   def _qcsapi(self, *command):
-    return subprocess.check_output(['qcsapi'] + list(command)).strip()
+    try:
+      return subprocess.check_output(['qcsapi'] + list(command)).strip()
+    except subprocess.CalledProcessError as e:
+      logging.error('QCSAPI call failed: %s: %s', e, e.output)
+      raise
 
   def attach(self):
     self._update()