bpo-24758: Improve the error msg for unittest.mock.Mock()'s unsafe mode (#12991)

* bpo-24758: Improve the error msg for unittest.mock.Mock()'s unsafe mode

* Make the requested changes.

Lib/unittest/mock.py

@@ -572,7 +572,8 @@ class NonCallableMock(Base):
             raise AttributeError(name)
         if not self._mock_unsafe:
             if name.startswith(('assert', 'assret')):
-                raise AttributeError(name)
+                raise AttributeError("Attributes cannot start with 'assert' "
+                                     "or 'assret'")
 
         result = self._mock_children.get(name)
         if result is _deleted: