fix test_supervision_with_sending_error flaky test (#767)

Summary:
Pull Request resolved: #767

The error message could be difference, but both are supervision error. Fixing
the test by checking the supervisione error prefix now.

Reviewed By: pzhan9

Differential Revision: D79668418

fbshipit-source-id: 6ebff39c820bd8284c4524300b2c153bbb8e1a7d

Author: moonli

python/tests/test_actor_error.py

@@ -662,9 +662,7 @@ async def test_supervision_with_sending_error():
     await actor_mesh.check_with_payload.call(payload="a")
 
     # send a large payload to trigger send timeout error
-    with pytest.raises(
-        SupervisionError, match="supervision error:.*actor mesh is stopped"
-    ):
+    with pytest.raises(SupervisionError, match="supervision error:.*"):
         await actor_mesh.check_with_payload.call(payload="a" * 55000000)
 
     # new call should fail with check of health state of actor mesh