Add new remote error introspection attrs

To handle both remote cancellation this adds `ContextCanceled.canceller:
tuple` the uid of the cancel requesting actor and is expected to be set
by the runtime when servicing any remote cancel request. This makes it
possible for `ContextCancelled` receivers to know whether "their actor
runtime" is the source of the cancellation.

Also add an explicit `RemoteActor.src_actor_uid` which better formalizes
the notion of "which remote actor" the error originated from.

Both of these new attrs are expected to be packed in the `.msgdata` when
the errors are loaded locally.

tractor/_exceptions.py

@@ -18,18 +18,18 @@
 Our classy exception set.
 
 """
+import builtins
+import importlib
 from typing import (
     Any,
-    Optional,
     Type,
 )
-import importlib
-import builtins
 import traceback
 
 import exceptiongroup as eg
 import trio
 
+from ._state import current_actor
 
 _this_mod = importlib.import_module(__name__)
 
@@ -44,7 +44,7 @@ class RemoteActorError(Exception):
     def __init__(
         self,
         message: str,
-        suberror_type: Optional[Type[BaseException]] = None,
+        suberror_type: Type[BaseException] | None = None,
         **msgdata
 
     ) -> None:
@@ -53,6 +53,10 @@ class RemoteActorError(Exception):
         self.type = suberror_type
         self.msgdata = msgdata
 
+    @property
+    def src_actor_uid(self) -> tuple[str, str] | None:
+        return self.msgdata.get('src_actor_uid')
+
 
 class InternalActorError(RemoteActorError):
     '''
@@ -62,14 +66,21 @@ class InternalActorError(RemoteActorError):
     '''
 
 
+class ContextCancelled(RemoteActorError):
+    '''
+    Inter-actor task context was cancelled by either a call to
+    ``Portal.cancel_actor()`` or ``Context.cancel()``.
+
+    '''
+    @property
+    def canceller(self) -> tuple[str, str] | None:
+        return self.msgdata.get('canceller')
+
+
 class TransportClosed(trio.ClosedResourceError):
     "Underlying channel transport was closed prior to use"
 
 
-class ContextCancelled(RemoteActorError):
-    "Inter-actor task context cancelled itself on the callee side."
-
-
 class NoResult(RuntimeError):
     "No final result is expected for this actor"
 
@@ -108,13 +119,17 @@ def pack_error(
     else:
         tb_str = traceback.format_exc()
 
-    return {
+    error_msg = {
-        'error': {
+        'tb_str': tb_str,
-            'tb_str': tb_str,
+        'type_str': type(exc).__name__,
-            'type_str': type(exc).__name__,
+        'src_actor_uid': current_actor().uid,
-        }
     }
 
+    if isinstance(exc, ContextCancelled):
+        error_msg.update(exc.msgdata)
+
+    return {'error': error_msg}
+
 
 def unpack_error(