Fail early if extension doesn't implement setup()

Fixes #813

docs/changelog.rst

@@ -20,6 +20,10 @@ Bug fix release.
 
 - Zeroconf: Fix intermittent DBus/Avahi exception.
 
+- Extensions: Fail early if trying to setup an extension which doesn't
+  implement the :meth:`mopidy.ext.Extension.setup` method. (Fixes:
+  :issue:`813`)
+
 
 v0.19.2 (2014-07-26)
 ====================

mopidy/ext.py

@@ -99,7 +99,7 @@ class Extension(object):
         :param registry: the extension registry
         :type registry: :class:`Registry`
         """
-        pass
+        raise NotImplementedError
 
 
 class Registry(collections.Mapping):

tests/test_ext.py

@@ -19,7 +19,8 @@ class ExtensionTest(unittest.TestCase):
         self.assertIsNone(self.ext.version)
 
     def test_get_default_config_raises_not_implemented(self):
-        self.assertRaises(NotImplementedError, self.ext.get_default_config)
+        with self.assertRaises(NotImplementedError):
+            self.ext.get_default_config()
 
     def test_get_config_schema_returns_extension_schema(self):
         schema = self.ext.get_config_schema()
@@ -27,3 +28,7 @@ class ExtensionTest(unittest.TestCase):
 
     def test_validate_environment_does_nothing_by_default(self):
         self.assertIsNone(self.ext.validate_environment())
+
+    def test_setup_raises_not_implemented(self):
+        with self.assertRaises(NotImplementedError):
+            self.ext.setup(None)