Add more explanation to missing python actor object error

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.

Differential Revision: D77446127

Author: pzhan9

python/monarch/actor_mesh.py

@@ -522,7 +522,22 @@ async def handle_cast(
 
             if self.instance is None:
                 raise AssertionError(
-                    "__init__ failed earlier and no Actor object is available"
+                    f"""
+                    Actor object is not available when executing method {message.method}
+                    on actor {mailbox.actor_id}. This could happen because:
+                    1. the execution of previous message for "__init__" failed;
+                    2. this message is delivered to this actor before the previous
+                       message of "__init__" is delivered. Normally out-of-order
+                       delivery should not happen. But it is possible if both
+                       "cast" and "direct actor send" are used. For example
+                       "__init__" is cast, and this message is direct send. Then
+                       this message could be delivered earlier since we do not
+                       guarrantee ordering of delivery interleaved by these 2
+                       usages.
+                         * Since we do not expose the choice of "cast" and "direct
+                           send" to the user, when this error happens, it is
+                           likely a bug in the framework.
+                    """
                 )
             the_method = getattr(self.instance, message.method)._method