jsonrpc: Explain why call to private method failed

mopidy/utils/jsonrpc.py

@@ -206,9 +206,10 @@ class JsonRpcWrapper(object):
             mount, method_name = method_path.rsplit('.', 1)
         else:
             mount, method_name = '', method_path
+        if method_name.startswith('_'):
+            raise JsonRpcMethodNotFoundError(
+                data='Private methods are not exported')
         try:
-            if method_name.startswith('_'):
-                raise AttributeError
             obj = self.objects[mount]
             return getattr(obj, method_name)
         except (AttributeError, KeyError):

tests/utils/jsonrpc_test.py

@@ -405,6 +405,7 @@ class JsonRpcSingleCommandErrorTest(JsonRpcTestBase):
         error = response['error']
         self.assertEqual(error['code'], -32601)
         self.assertEqual(error['message'], 'Method not found')
+        self.assertEqual(error['data'], 'Private methods are not exported')
 
     def test_invalid_params_causes_invalid_params_error(self):
         request = {