fix error msg, add test

Author: roveo

streamz/core.py

@@ -312,7 +312,7 @@ def stub(*args, **kwargs):
                 raise TypeError(
                     f"Error loading {entry_point.name} "
                     f"from module {entry_point.module_name}: "
-                    f"{entry_point.cls.__name__} must be a subclass of Stream"
+                    f"{node.__class__.__name__} must be a subclass of Stream"
                 )
             cls.register_api(
                 modifier=modifier, attribute_name=entry_point.name

streamz/tests/test_plugins.py

@@ -53,3 +53,17 @@ class invalid_node:
 
     with pytest.raises(TypeError):
         Stream.test()
+
+
+def test_register_plugin_entry_point_already_registered():
+    @Stream.register_api()
+    class test(Stream):
+        pass
+
+    entry_point = MockEntryPoint("test_double", test, "test_module")
+
+    Stream.register_plugin_entry_point(entry_point)
+
+    assert Stream.test_double.__name__ == "stub"
+    Stream.test_double()
+    assert Stream.test_double.__name__ == "test"