py3: Avoid indexing exception objects

It doesn't work on Python 3.

tests/mpd/test_exceptions.py

@@ -13,7 +13,7 @@ class MpdExceptionsTest(unittest.TestCase):
             try:
                 raise KeyError('Track X not found')
             except KeyError as e:
-                raise MpdAckError(e[0])
+                raise MpdAckError(e.message)
         except MpdAckError as e:
             self.assertEqual(e.message, 'Track X not found')