dbuslib: Fix UnknownInterface error

The string "org.freedesktop.UnknownInterface" is not an
existing dbus error string, all errors are of format
*.DBus.Error.* so change this error string to the right one.

Also fix the error responses when dbus errors are raised.

Part of openbmc/openbmc#2463

Change-Id: I384393a6f81c5057173e2941cd109e2f952e1311
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/obmc/dbuslib/enums.py b/obmc/dbuslib/enums.py
index 4ddec42..f1d4d64 100644
--- a/obmc/dbuslib/enums.py
+++ b/obmc/dbuslib/enums.py
@@ -17,7 +17,7 @@
 import dbus
 
 DBUS_OBJMGR_IFACE = dbus.BUS_DAEMON_IFACE + '.ObjectManager'
-DBUS_UNKNOWN_INTERFACE = 'org.freedesktop.UnknownInterface'
+DBUS_UNKNOWN_INTERFACE = 'org.freedesktop.DBus.Error.UnknownInterface'
 DBUS_UNKNOWN_SERVICE = 'org.freedesktop.DBus.Error.ServiceUnknown'
 DBUS_UNKNOWN_PROPERTY = 'org.freedesktop.DBus.Error.UnknownProperty'
 DBUS_UNKNOWN_METHOD = 'org.freedesktop.DBus.Error.UnknownMethod'