Add more explanation to missing python actor object error (#366)

Summary:

I hit this error when plumbing `cast` to the python side (D77386342, v1). Briefly, I use `cast` [when mesh is available, and fall back on direct send if not](https://www.internalfb.com/code/fbsource/[D77386342-V1]/fbcode/monarch/python/monarch/actor_mesh.py?lines=192-216). This lead to a mixture usage of `cast` and `direct send` in this test

https://www.internalfb.com/code/fbsource/[5ef41e9fdc95c3a2f4f9135244dbb990d0818d0d]/fbcode/monarch/python/meta/tests/test_python_actors.py?lines=256-263

Briefly, `proc.spawn` uses cast, but inside `f.get.stream(t)`, it uses direct send:

https://www.internalfb.com/code/fbsource/[5ef41e9fdc95c3a2f4f9135244dbb990d0818d0d]/fbcode/monarch/python/meta/tests/test_python_actors.py?lines=250-253

That is because when `t` gets pickle, `PythonActorMesh` is [not included](https://www.internalfb.com/code/fbsource/[5ef41e9fdc95c3a2f4f9135244dbb990d0818d0d]/fbcode/monarch/python/monarch/actor_mesh.py?lines=153-163).

This is very painful to debug. Let us beef up the error message so hopefully I can remember what happened when I run into it next time.

Reviewed By: mariusae

Differential Revision: D77446127

Author: pzhan9

python/monarch/actor_mesh.py

@@ -521,8 +521,21 @@ async def handle_cast(
                 return None
 
             if self.instance is None:
+                # This could happen because of the following reasons. Both
+                # indicates a possible bug in the framework:
+                # 1. the execution of the previous message for "__init__" failed,
+                #    but that error is not surfaced to the caller.
+                #      - TODO(T229200522): there is a known bug. fix it.
+                # 2. this message is delivered to this actor before the previous
+                #    message of "__init__" is delivered. Out-of-order delivery
+                #    should never happen. It indicates either a bug in the
+                #    message delivery mechanism, or the framework accidentally
+                #    mixed the usage of cast and direct send.
                 raise AssertionError(
-                    "__init__ failed earlier and no Actor object is available"
+                    f"""
+                    actor object is missing when executing method {message.method}
+                    on actor {mailbox.actor_id}
+                    """
                 )
             the_method = getattr(self.instance, message.method)._method