Handle differing dbus exception implementations

sdbus and python-dbus don't report the unknown interface
exception the same way, so add a workaround.
diff --git a/obmc-rest b/obmc-rest
index 3a36406..226b1b7 100644
--- a/obmc-rest
+++ b/obmc-rest
@@ -34,6 +34,7 @@
 import crypt
 
 DBUS_UNKNOWN_INTERFACE = 'org.freedesktop.UnknownInterface'
+DBUS_UNKNOWN_INTERFACE_ERROR = 'org.freedesktop.DBus.Error.UnknownInterface'
 DBUS_UNKNOWN_METHOD = 'org.freedesktop.DBus.Error.UnknownMethod'
 DBUS_INVALID_ARGS = 'org.freedesktop.DBus.Error.InvalidArgs'
 DBUS_TYPE_ERROR = 'org.freedesktop.DBus.Python.TypeError'
@@ -130,6 +131,9 @@
             if DBUS_UNKNOWN_INTERFACE in e.get_dbus_message():
                 # interface doesn't have any properties
                 return None
+            if DBUS_UNKNOWN_INTERFACE_ERROR in e.get_dbus_name():
+                # interface doesn't have any properties
+                return None
             if DBUS_UNKNOWN_METHOD == e.get_dbus_name():
                 # properties interface not implemented at all
                 return None